cmd/cue: fix deduplication of message in vet

Change-Id: I35dba3f0f9bf8479678d869470330a400e5a2b8f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2377
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cmd/cue/cmd/vet.go b/cmd/cue/cmd/vet.go
index a7bbacc..a55a5aa 100644
--- a/cmd/cue/cmd/vet.go
+++ b/cmd/cue/cmd/vet.go
@@ -47,6 +47,8 @@
 		exprs = append(exprs, expr)
 	}
 
+	shown := false
+
 	for _, inst := range instances {
 		// TODO: use ImportPath or some other sanitized path.
 
@@ -64,7 +66,6 @@
 			cue.Hidden(true),
 		}
 		w := cmd.OutOrStderr()
-		shown := false
 		err := inst.Value().Validate(append(opt, cue.Concrete(concrete))...)
 		if err != nil && !hasFlag {
 			err = inst.Value().Validate(append(opt, cue.Concrete(false))...)