diff --git a/asm/main.c b/asm/main.c
index 071d23d..e2e0737 100644
--- a/asm/main.c
+++ b/asm/main.c
@@ -260,6 +260,12 @@ static inline Args parse_args(int argc, char** argv)
                 REPORTF_ERROR("%s", "no filename given to -o");
                 exit(1);
             }
+            if (output_file != NULL) {
+                REPORTF_ERROR(
+                    "multiple output filenames specified, original was '%s'",
+                    output_file);
+                exit(1);
+            }
             output_file = argv[i];
         } else {
             if (input_file != NULL) {