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_test.go

@@ -0,0 +1,28 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package issues_test
+
+import (
+	"testing"
+
+	issues_model "code.gitea.io/gitea/models/issues"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewIssueLabelsScope(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	issue := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 18})
+	label1 := unittest.AssertExistsAndLoadBean(t, &issues_model.Label{ID: 7})
+	label2 := unittest.AssertExistsAndLoadBean(t, &issues_model.Label{ID: 8})
+	doer := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+
+	assert.NoError(t, issues_model.NewIssueLabels(issue, []*issues_model.Label{label1, label2}, doer))
+
+	assert.Len(t, issue.Labels, 1)
+	assert.Equal(t, label2.ID, issue.Labels[0].ID)
+}