Don't let API add 2 exclusive labels from same scope (#27433) (#27460)

Backport #27433 by @JakobDev

Fixes #27380

Co-authored-by: JakobDev <jakobdev@gmx.de>

models/issues/issue_label.go

@@ -54,6 +54,8 @@ func newIssueLabel(ctx context.Context, issue *Issue, label *Label, doer *user_m
 		return err
 	}
 
+	issue.Labels = append(issue.Labels, label)
+
 	return updateLabelCols(ctx, label, "num_issues", "num_closed_issue")
 }
 
@@ -122,6 +124,11 @@ func newIssueLabels(ctx context.Context, issue *Issue, labels []*Label, doer *us
 	if err = issue.LoadRepo(ctx); err != nil {
 		return err
 	}
+
+	if err = issue.LoadLabels(ctx); err != nil {
+		return err
+	}
+
 	for _, l := range labels {
 		// Don't add already present labels and invalid labels
 		if HasIssueLabel(ctx, issue.ID, l.ID) ||
@@ -129,6 +136,10 @@ func newIssueLabels(ctx context.Context, issue *Issue, labels []*Label, doer *us
 			continue
 		}
 
+		if err = RemoveDuplicateExclusiveIssueLabels(ctx, issue, l, doer); err != nil {
+			return err
+		}
+
 		if err = newIssueLabel(ctx, issue, l, doer); err != nil {
 			return fmt.Errorf("newIssueLabel: %w", err)
 		}