FIXME removal: This is an appropriate error code already.

src/physfs.c

@@ -1431,7 +1431,7 @@ static int doRegisterArchiver(const PHYSFS_Archiver *_archiver)
     for (i = 0; i < numArchivers; i++)
     {
         if (__PHYSFS_utf8stricmp(archiveInfo[i]->extension, ext) == 0)
-            BAIL(PHYSFS_ERR_DUPLICATE, 0);  /* !!! FIXME: better error? ERR_IN_USE? */
+            BAIL(PHYSFS_ERR_DUPLICATE, 0);
     } /* for */
 
     /* make a copy of the data. */