1 files changed, 5 insertions, 1 deletions

diff --git a/lib-src/make-docfile.c b/lib-src/make-docfile.c
index df9c6e069f5..006600e162d 100644
--- a/lib-src/make-docfile.c
+++ b/lib-src/make-docfile.c
@@ -322,8 +322,9 @@ scan_c_file (filename, mode)
   register int defvarperbufferflag;
   register int defvarflag;
   int minargs, maxargs;
+  int extension = filename[strlen (filename) - 1];
 
-  if (filename[strlen (filename) - 1] == 'o')
+  if (extension == 'o')
     filename[strlen (filename) - 1] = 'c';
 
   infile = fopen (filename, mode);
@@ -335,6 +336,9 @@ scan_c_file (filename, mode)
       return 0;
     }
 
+  /* Reset extension to be able to detect duplicate files. */
+  filename[strlen (filename) - 1] = extension;
+
   c = '\n';
   while (!feof (infile))
     {