diff --git a/cc/gen_stub_libs.py b/cc/gen_stub_libs.py
index 32765a7..c49d197 100755
--- a/cc/gen_stub_libs.py
+++ b/cc/gen_stub_libs.py
@@ -20,6 +20,7 @@
 import logging
 import os
 import re
+import sys
 
 
 ALL_ARCHITECTURES = (
@@ -107,22 +108,33 @@
     return version.endswith('_PRIVATE') or version.endswith('_PLATFORM')
 
 
-def should_omit_version(name, tags, arch, api, vndk):
+def should_omit_version(version, arch, api, vndk):
     """Returns True if the version section should be ommitted.
 
     We want to omit any sections that do not have any symbols we'll have in the
     stub library. Sections that contain entirely future symbols or only symbols
     for certain architectures.
     """
-    if version_is_private(name):
+    if version_is_private(version.name):
         return True
-    if 'platform-only' in tags:
+    if 'platform-only' in version.tags:
         return True
-    if 'vndk' in tags and not vndk:
+    if 'vndk' in version.tags and not vndk:
         return True
-    if not symbol_in_arch(tags, arch):
+    if not symbol_in_arch(version.tags, arch):
         return True
-    if not symbol_in_api(tags, arch, api):
+    if not symbol_in_api(version.tags, arch, api):
+        return True
+    return False
+
+
+def should_omit_symbol(symbol, arch, api, vndk):
+    """Returns True if the symbol should be omitted."""
+    if not vndk and 'vndk' in symbol.tags:
+        return True
+    if not symbol_in_arch(symbol.tags, arch):
+        return True
+    if not symbol_in_api(symbol.tags, arch, api):
         return True
     return False
 
@@ -189,6 +201,15 @@
     pass
 
 
+class MultiplyDefinedSymbolError(RuntimeError):
+    """A symbol name was multiply defined."""
+    def __init__(self, multiply_defined_symbols):
+        super(MultiplyDefinedSymbolError, self).__init__(
+            'Version script contains multiple definitions for: {}'.format(
+                ', '.join(multiply_defined_symbols)))
+        self.multiply_defined_symbols = multiply_defined_symbols
+
+
 class Version(object):
     """A version block of a symbol file."""
     def __init__(self, name, base, tags, symbols):
@@ -221,9 +242,12 @@
 
 class SymbolFileParser(object):
     """Parses NDK symbol files."""
-    def __init__(self, input_file, api_map):
+    def __init__(self, input_file, api_map, arch, api, vndk):
         self.input_file = input_file
         self.api_map = api_map
+        self.arch = arch
+        self.api = api
+        self.vndk = vndk
         self.current_line = None
 
     def parse(self):
@@ -235,8 +259,36 @@
             else:
                 raise ParseError(
                     'Unexpected contents at top level: ' + self.current_line)
+
+        self.check_no_duplicate_symbols(versions)
         return versions
 
+    def check_no_duplicate_symbols(self, versions):
+        """Raises errors for multiply defined symbols.
+
+        This situation is the normal case when symbol versioning is actually
+        used, but this script doesn't currently handle that. The error message
+        will be a not necessarily obvious "error: redefition of 'foo'" from
+        stub.c, so it's better for us to catch this situation and raise a
+        better error.
+        """
+        symbol_names = set()
+        multiply_defined_symbols = set()
+        for version in versions:
+            if should_omit_version(version, self.arch, self.api, self.vndk):
+                continue
+
+            for symbol in version.symbols:
+                if should_omit_symbol(symbol, self.arch, self.api, self.vndk):
+                    continue
+
+                if symbol.name in symbol_names:
+                    multiply_defined_symbols.add(symbol.name)
+                symbol_names.add(symbol.name)
+        if multiply_defined_symbols:
+            raise MultiplyDefinedSymbolError(
+                sorted(list(multiply_defined_symbols)))
+
     def parse_version(self):
         """Parses a single version section and returns a Version object."""
         name = self.current_line.split('{')[0].strip()
@@ -325,20 +377,14 @@
 
     def write_version(self, version):
         """Writes a single version block's data to the output files."""
-        name = version.name
-        tags = version.tags
-        if should_omit_version(name, tags, self.arch, self.api, self.vndk):
+        if should_omit_version(version, self.arch, self.api, self.vndk):
             return
 
-        section_versioned = symbol_versioned_in_api(tags, self.api)
+        section_versioned = symbol_versioned_in_api(version.tags, self.api)
         version_empty = True
         pruned_symbols = []
         for symbol in version.symbols:
-            if not self.vndk and 'vndk' in symbol.tags:
-                continue
-            if not symbol_in_arch(symbol.tags, self.arch):
-                continue
-            if not symbol_in_api(symbol.tags, self.arch, self.api):
+            if should_omit_symbol(symbol, self.arch, self.api, self.vndk):
                 continue
 
             if symbol_versioned_in_api(symbol.tags, self.api):
@@ -433,7 +479,11 @@
     logging.basicConfig(level=verbose_map[verbosity])
 
     with open(args.symbol_file) as symbol_file:
-        versions = SymbolFileParser(symbol_file, api_map).parse()
+        try:
+            versions = SymbolFileParser(symbol_file, api_map, args.arch, api,
+                                        args.vndk).parse()
+        except MultiplyDefinedSymbolError as ex:
+            sys.exit('{}: error: {}'.format(args.symbol_file, ex))
 
     with open(args.stub_src, 'w') as src_file:
         with open(args.version_script, 'w') as version_file:
diff --git a/cc/test_gen_stub_libs.py b/cc/test_gen_stub_libs.py
index 279ef52..3b5585a 100755
--- a/cc/test_gen_stub_libs.py
+++ b/cc/test_gen_stub_libs.py
@@ -163,39 +163,94 @@
 
 class OmitVersionTest(unittest.TestCase):
     def test_omit_private(self):
-        self.assertFalse(gsl.should_omit_version('foo', [], 'arm', 9, False))
-
-        self.assertTrue(gsl.should_omit_version(
-            'foo_PRIVATE', [], 'arm', 9, False))
-        self.assertTrue(gsl.should_omit_version(
-            'foo_PLATFORM', [], 'arm', 9, False))
-
-        self.assertTrue(gsl.should_omit_version(
-            'foo', ['platform-only'], 'arm', 9, False))
-
-    def test_omit_vndk(self):
-        self.assertTrue(gsl.should_omit_version(
-            'foo', ['vndk'], 'arm', 9, False))
-
-        self.assertFalse(gsl.should_omit_version('foo', [], 'arm', 9, True))
-        self.assertFalse(gsl.should_omit_version(
-            'foo', ['vndk'], 'arm', 9, True))
-
-    def test_omit_arch(self):
-        self.assertFalse(gsl.should_omit_version('foo', [], 'arm', 9, False))
-        self.assertFalse(gsl.should_omit_version(
-            'foo', ['arm'], 'arm', 9, False))
-
-        self.assertTrue(gsl.should_omit_version(
-            'foo', ['x86'], 'arm', 9, False))
-
-    def test_omit_api(self):
-        self.assertFalse(gsl.should_omit_version('foo', [], 'arm', 9, False))
         self.assertFalse(
-            gsl.should_omit_version('foo', ['introduced=9'], 'arm', 9, False))
+            gsl.should_omit_version(
+                gsl.Version('foo', None, [], []), 'arm', 9, False))
 
         self.assertTrue(
-            gsl.should_omit_version('foo', ['introduced=14'], 'arm', 9, False))
+            gsl.should_omit_version(
+                gsl.Version('foo_PRIVATE', None, [], []), 'arm', 9, False))
+        self.assertTrue(
+            gsl.should_omit_version(
+                gsl.Version('foo_PLATFORM', None, [], []), 'arm', 9, False))
+
+        self.assertTrue(
+            gsl.should_omit_version(
+                gsl.Version('foo', None, ['platform-only'], []), 'arm', 9,
+                False))
+
+    def test_omit_vndk(self):
+        self.assertTrue(
+            gsl.should_omit_version(
+                gsl.Version('foo', None, ['vndk'], []), 'arm', 9, False))
+
+        self.assertFalse(
+            gsl.should_omit_version(
+                gsl.Version('foo', None, [], []), 'arm', 9, True))
+        self.assertFalse(
+            gsl.should_omit_version(
+                gsl.Version('foo', None, ['vndk'], []), 'arm', 9, True))
+
+    def test_omit_arch(self):
+        self.assertFalse(
+            gsl.should_omit_version(
+                gsl.Version('foo', None, [], []), 'arm', 9, False))
+        self.assertFalse(
+            gsl.should_omit_version(
+                gsl.Version('foo', None, ['arm'], []), 'arm', 9, False))
+
+        self.assertTrue(
+            gsl.should_omit_version(
+                gsl.Version('foo', None, ['x86'], []), 'arm', 9, False))
+
+    def test_omit_api(self):
+        self.assertFalse(
+            gsl.should_omit_version(
+                gsl.Version('foo', None, [], []), 'arm', 9, False))
+        self.assertFalse(
+            gsl.should_omit_version(
+                gsl.Version('foo', None, ['introduced=9'], []), 'arm', 9,
+                False))
+
+        self.assertTrue(
+            gsl.should_omit_version(
+                gsl.Version('foo', None, ['introduced=14'], []), 'arm', 9,
+                False))
+
+
+class OmitSymbolTest(unittest.TestCase):
+    def test_omit_vndk(self):
+        self.assertTrue(
+            gsl.should_omit_symbol(
+                gsl.Symbol('foo', ['vndk']), 'arm', 9, False))
+
+        self.assertFalse(
+            gsl.should_omit_symbol(gsl.Symbol('foo', []), 'arm', 9, True))
+        self.assertFalse(
+            gsl.should_omit_symbol(
+                gsl.Symbol('foo', ['vndk']), 'arm', 9, True))
+
+    def test_omit_arch(self):
+        self.assertFalse(
+            gsl.should_omit_symbol(gsl.Symbol('foo', []), 'arm', 9, False))
+        self.assertFalse(
+            gsl.should_omit_symbol(
+                gsl.Symbol('foo', ['arm']), 'arm', 9, False))
+
+        self.assertTrue(
+            gsl.should_omit_symbol(
+                gsl.Symbol('foo', ['x86']), 'arm', 9, False))
+
+    def test_omit_api(self):
+        self.assertFalse(
+            gsl.should_omit_symbol(gsl.Symbol('foo', []), 'arm', 9, False))
+        self.assertFalse(
+            gsl.should_omit_symbol(
+                gsl.Symbol('foo', ['introduced=9']), 'arm', 9, False))
+
+        self.assertTrue(
+            gsl.should_omit_symbol(
+                gsl.Symbol('foo', ['introduced=14']), 'arm', 9, False))
 
 
 class SymbolFileParseTest(unittest.TestCase):
@@ -207,7 +262,7 @@
             # baz
             qux
         """))
-        parser = gsl.SymbolFileParser(input_file, {})
+        parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
         self.assertIsNone(parser.current_line)
 
         self.assertEqual('foo', parser.next_line().strip())
@@ -232,7 +287,7 @@
             VERSION_2 {
             } VERSION_1; # asdf
         """))
-        parser = gsl.SymbolFileParser(input_file, {})
+        parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
 
         parser.next_line()
         version = parser.parse_version()
@@ -256,7 +311,7 @@
         input_file = io.StringIO(textwrap.dedent("""\
             VERSION_1 {
         """))
-        parser = gsl.SymbolFileParser(input_file, {})
+        parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
         parser.next_line()
         with self.assertRaises(gsl.ParseError):
             parser.parse_version()
@@ -267,7 +322,7 @@
                 foo:
             }
         """))
-        parser = gsl.SymbolFileParser(input_file, {})
+        parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
         parser.next_line()
         with self.assertRaises(gsl.ParseError):
             parser.parse_version()
@@ -277,7 +332,7 @@
             foo;
             bar; # baz qux
         """))
-        parser = gsl.SymbolFileParser(input_file, {})
+        parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
 
         parser.next_line()
         symbol = parser.parse_symbol()
@@ -295,7 +350,7 @@
                 *;
             };
         """))
-        parser = gsl.SymbolFileParser(input_file, {})
+        parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
         parser.next_line()
         with self.assertRaises(gsl.ParseError):
             parser.parse_version()
@@ -307,7 +362,7 @@
                     *;
             };
         """))
-        parser = gsl.SymbolFileParser(input_file, {})
+        parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
         parser.next_line()
         version = parser.parse_version()
         self.assertEqual([], version.symbols)
@@ -318,7 +373,7 @@
                 foo
             };
         """))
-        parser = gsl.SymbolFileParser(input_file, {})
+        parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
         parser.next_line()
         with self.assertRaises(gsl.ParseError):
             parser.parse_version()
@@ -326,7 +381,7 @@
     def test_parse_fails_invalid_input(self):
         with self.assertRaises(gsl.ParseError):
             input_file = io.StringIO('foo')
-            parser = gsl.SymbolFileParser(input_file, {})
+            parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
             parser.parse()
 
     def test_parse(self):
@@ -347,7 +402,7 @@
                     qwerty;
             } VERSION_1;
         """))
-        parser = gsl.SymbolFileParser(input_file, {})
+        parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
         versions = parser.parse()
 
         expected = [
@@ -505,7 +560,7 @@
                 wobble;
             } VERSION_4;
         """))
-        parser = gsl.SymbolFileParser(input_file, api_map)
+        parser = gsl.SymbolFileParser(input_file, api_map, 'arm', 9, False)
         versions = parser.parse()
 
         src_file = io.StringIO()
@@ -555,7 +610,7 @@
                     *;
             };
         """))
-        parser = gsl.SymbolFileParser(input_file, api_map)
+        parser = gsl.SymbolFileParser(input_file, api_map, 'arm', 9001, False)
         versions = parser.parse()
 
         src_file = io.StringIO()
@@ -578,6 +633,38 @@
         """)
         self.assertEqual(expected_version, version_file.getvalue())
 
+    def test_multiple_definition(self):
+        input_file = io.StringIO(textwrap.dedent("""\
+            VERSION_1 {
+                global:
+                    foo;
+                    foo;
+                    bar;
+                    baz;
+                    qux; # arm
+                local:
+                    *;
+            };
+
+            VERSION_2 {
+                global:
+                    bar;
+                    qux; # arm64
+            } VERSION_1;
+
+            VERSION_PRIVATE {
+                global:
+                    baz;
+            } VERSION_2;
+
+        """))
+        parser = gsl.SymbolFileParser(input_file, {}, 'arm', 16, False)
+
+        with self.assertRaises(gsl.MultiplyDefinedSymbolError) as cm:
+            parser.parse()
+        self.assertEquals(['bar', 'foo'],
+                          cm.exception.multiply_defined_symbols)
+
 
 def main():
     suite = unittest.TestLoader().loadTestsFromName(__name__)