diff --git a/src/errors.h b/src/errors.h
index cc0af8a..bf99433 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3489,6 +3489,10 @@
 
 EXTERN char e_interface_str_and_class_str_function_access_not_same[]
 	INIT(= N_("E1367: Access type of class method %s differs from interface method %s"));
+EXTERN char e_static_cannot_be_followed_by_this[]
+	INIT(= N_("E1368: Static cannot be followed by \"this\" in a member name"));
+EXTERN char e_duplicate_member_str[]
+	INIT(= N_("E1369: Duplicate member: %s"));
 EXTERN char e_cannot_mix_positional_and_non_positional_str[]
 	INIT(= N_("E1400: Cannot mix positional and non-positional arguments: %s"));
 EXTERN char e_fmt_arg_nr_unused_str[]
diff --git a/src/testdir/test_vim9_class.vim b/src/testdir/test_vim9_class.vim
index f7d6b73..319eb05 100644
--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -894,6 +894,15 @@
     endclass
   END
   v9.CheckScriptFailure(lines, 'E1065:')
+
+  # Test for "static" cannot be followed by "this".
+  lines =<< trim END
+    vim9script
+    class Something
+      static this.val = 1
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1368: Static cannot be followed by "this" in a member name')
 enddef
 
 def Test_class_object_compare()
@@ -3438,4 +3447,92 @@
   v9.CheckScriptSuccess(lines)
 enddef
 
+" Test for declaring duplicate object and class members
+def Test_dup_member_variable()
+  # Duplicate member variable
+  var lines =<< trim END
+    vim9script
+    class C
+      this.val = 10
+      this.val = 20
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1369: Duplicate member: val')
+
+  # Duplicate private member variable
+  lines =<< trim END
+    vim9script
+    class C
+      this._val = 10
+      this._val = 20
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1369: Duplicate member: _val')
+
+  # Duplicate public member variable
+  lines =<< trim END
+    vim9script
+    class C
+      public this.val = 10
+      public this.val = 20
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1369: Duplicate member: val')
+
+  # Duplicate private member variable
+  lines =<< trim END
+    vim9script
+    class C
+      this.val = 10
+      this._val = 20
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1369: Duplicate member: _val')
+
+  # Duplicate public and private member variable
+  lines =<< trim END
+    vim9script
+    class C
+      this._val = 20
+      public this.val = 10
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1369: Duplicate member: val')
+
+  # Duplicate class member variable
+  lines =<< trim END
+    vim9script
+    class C
+      static s: string = "abc"
+      static _s: string = "def"
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1369: Duplicate member: _s')
+
+  # Duplicate public and private class member variable
+  lines =<< trim END
+    vim9script
+    class C
+      public static s: string = "abc"
+      static _s: string = "def"
+    endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1369: Duplicate member: _s')
+
+  # Duplicate class and object member variable
+  lines =<< trim END
+    vim9script
+    class C
+      static val = 10
+      this.val = 20
+      def new()
+      enddef
+    endclass
+    var c = C.new()
+    assert_equal(10, C.val)
+    assert_equal(20, c.val)
+  END
+  v9.CheckScriptSuccess(lines)
+enddef
+
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker
diff --git a/src/version.c b/src/version.c
index 7b8683e..ce28694 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1814,
+/**/
     1813,
 /**/
     1812,
diff --git a/src/vim9class.c b/src/vim9class.c
index 36a5d0a..955bd0c 100644
--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -24,7 +24,7 @@
 /*
  * Parse a member declaration, both object and class member.
  * Returns OK or FAIL.  When OK then "varname_end" is set to just after the
- * variable name and "type_ret" is set to the decleared or detected type.
+ * variable name and "type_ret" is set to the declared or detected type.
  * "init_expr" is set to the initialisation expression (allocated), if there is
  * one.  For an interface "init_expr" is NULL.
  */
@@ -490,6 +490,52 @@
 }
 
 /*
+ * Returns TRUE if the member "varname" is already defined.
+ */
+    static int
+is_duplicate_member(garray_T *mgap, char_u *varname, char_u *varname_end)
+{
+    char_u *pstr = (*varname == '_') ? varname + 1 : varname;
+
+    for (int i = 0; i < mgap->ga_len; ++i)
+    {
+	ocmember_T *m = ((ocmember_T *)mgap->ga_data) + i;
+	char_u *qstr = *m->ocm_name == '_' ? m->ocm_name + 1 : m->ocm_name;
+	if (STRNCMP(pstr, qstr, varname_end - pstr) == 0)
+	{
+	    char_u *name = vim_strnsave(varname, varname_end - varname);
+	    semsg(_(e_duplicate_member_str), name);
+	    vim_free(name);
+	    return TRUE;
+	}
+    }
+
+    return FALSE;
+}
+
+/*
+ * Returns TRUE if the method "name" is already defined.
+ */
+    static int
+is_duplicate_method(garray_T *fgap, char_u *name)
+{
+    char_u *pstr = (*name == '_') ? name + 1 : name;
+
+    for (int i = 0; i < fgap->ga_len; ++i)
+    {
+	char_u *n = ((ufunc_T **)fgap->ga_data)[i]->uf_name;
+	char_u *qstr = *n == '_' ? n + 1 : n;
+	if (STRCMP(pstr, qstr) == 0)
+	{
+	    semsg(_(e_duplicate_function_str), name);
+	    return TRUE;
+	}
+    }
+
+    return FALSE;
+}
+
+/*
  * Update the interface class lookup table for the member index on the
  * interface to the member index in the class implementing the interface.
  * And a lookup table for the object method index on the interface
@@ -1080,6 +1126,11 @@
 		semsg(_(e_invalid_object_member_declaration_str), p);
 		break;
 	    }
+	    if (has_static)
+	    {
+		emsg(_(e_static_cannot_be_followed_by_this));
+		break;
+	    }
 	    char_u *varname = p + 5;
 	    char_u *varname_end = NULL;
 	    type_T *type = NULL;
@@ -1088,6 +1139,11 @@
 			  &varname_end, &type_list, &type,
 			  is_class ? &init_expr: NULL) == FAIL)
 		break;
+	    if (is_duplicate_member(&objmembers, varname, varname_end))
+	    {
+		vim_free(init_expr);
+		break;
+	    }
 	    if (add_member(&objmembers, varname, varname_end,
 					  has_public, type, init_expr) == FAIL)
 	    {
@@ -1154,17 +1210,8 @@
 		garray_T *fgap = has_static || is_new
 					       ? &classfunctions : &objmethods;
 		// Check the name isn't used already.
-		for (int i = 0; i < fgap->ga_len; ++i)
-		{
-		    char_u *n = ((ufunc_T **)fgap->ga_data)[i]->uf_name;
-		    char_u *pstr = *name == '_' ? name + 1 : name;
-		    char_u *qstr = *n == '_' ? n + 1 : n;
-		    if (STRCMP(pstr, qstr) == 0)
-		    {
-			semsg(_(e_duplicate_function_str), name);
-			break;
-		    }
-		}
+		if (is_duplicate_method(fgap, name))
+		    break;
 
 		if (ga_grow(fgap, 1) == OK)
 		{
@@ -1197,6 +1244,11 @@
 		      &varname_end, &type_list, &type,
 		      is_class ? &init_expr : NULL) == FAIL)
 		break;
+	    if (is_duplicate_member(&classmembers, varname, varname_end))
+	    {
+		vim_free(init_expr);
+		break;
+	    }
 	    if (add_member(&classmembers, varname, varname_end,
 				      has_public, type, init_expr) == FAIL)
 	    {