--- cpp.c
+++ cpp.c
@@ -2464,7 +2464,7 @@
 			warning(-1, "unknown option '%s'", argv[i]);
 	} else {
 		if (filename != 0) {
-			error(-1, "spurious filename '%s'", argv[i]);
+			error(-1, "spurious duplicate filename '%s' - vs. '%s' ", argv[i], filename);
 			return 2;
 		}
 		filename = argv[i];