SUSv2 specifies that if the extension is identical to the basename, it

should not be removed.

coreutils/basename.c

@@ -44,7 +44,7 @@ extern int basename_main(int argc, char **argv)
 		argv++;
 		n = strlen(*argv);
 		m = strlen(s);
-		if (m>=n && strncmp(s+m-n, *argv, n)==0)
+		if (m>n && strncmp(s+m-n, *argv, n)==0)
 			s[m-n] = '\0';
 	}
 	puts(s);