diff --git a/src/lib/os.pl b/src/lib/os.pl
index 00c3e914b..cf3033943 100644
--- a/src/lib/os.pl
+++ b/src/lib/os.pl
@@ -138,9 +138,8 @@
     can_be(list, Argv),
     '$argv'(Argv0),
     ( member("--", Argv0) ->
-        append(Argv1, ["--"|Argv], Argv0),
-        \+ member("--", Argv1),
-        !
+        once(append(Argv1, ["--"|Argv], Argv0)),
+        \+ member("--", Argv1)
     ;
         Argv = []
     ).