patch 9.0.0865: duplicate arguments are not always detected

Problem:    Duplicate arguments are not always detected.
Solution:   Expand to full path before comparing arguments. (Nir Lichtman,
            closes #11505, closes #9402)
diff --git a/src/arglist.c b/src/arglist.c
index bc51cde..b35cdfe 100644
--- a/src/arglist.c
+++ b/src/arglist.c
@@ -784,9 +784,25 @@
     int j;
 
     for (i = 0; i < ARGCOUNT; ++i)
+    {
+	// Expand each argument to a full path to catch different paths leading
+	// to the same file.
+	char_u *firstFullname = FullName_save(ARGLIST[i].ae_fname, FALSE);
+	if (firstFullname == NULL)
+	    return;  // out of memory
+
 	for (j = i + 1; j < ARGCOUNT; ++j)
-	    if (fnamecmp(ARGLIST[i].ae_fname, ARGLIST[j].ae_fname) == 0)
+	{
+	    char_u *secondFullname = FullName_save(ARGLIST[j].ae_fname, FALSE);
+	    if (secondFullname == NULL)
+		break;  // out of memory
+	    int areNamesDuplicate =
+				  fnamecmp(firstFullname, secondFullname) == 0;
+	    vim_free(secondFullname);
+
+	    if (areNamesDuplicate)
 	    {
+		// remove one duplicate argument
 		vim_free(ARGLIST[j].ae_fname);
 		mch_memmove(ARGLIST + j, ARGLIST + j + 1,
 					(ARGCOUNT - j - 1) * sizeof(aentry_T));
@@ -799,6 +815,10 @@
 
 		--j;
 	    }
+	}
+
+	vim_free(firstFullname);
+    }
 }
 
 /*
diff --git a/src/testdir/test_arglist.vim b/src/testdir/test_arglist.vim
index 1bc556f..edc8b77 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -420,15 +420,19 @@
   call Reset_arglist()
   argdedupe
   call assert_equal([], argv())
+
   args a a a aa b b a b aa
   argdedupe
   call assert_equal(['a', 'aa', 'b'], argv())
+
   args a b c
   argdedupe
   call assert_equal(['a', 'b', 'c'], argv())
+
   args a
   argdedupe
   call assert_equal(['a'], argv())
+
   args a A b B
   argdedupe
   if has('fname_case')
@@ -436,11 +440,17 @@
   else
     call assert_equal(['a', 'b'], argv())
   endif
+
   args a b a c a b
   last
   argdedupe
   next
   call assert_equal('c', expand('%:t'))
+
+  args a ./a
+  argdedupe
+  call assert_equal(['a'], argv())
+
   %argd
 endfunc
 
diff --git a/src/version.c b/src/version.c
index a0c4371..98e7fd0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    865,
+/**/
     864,
 /**/
     863,