Cannot save topic if subtopic title already exist in another topic

[leechongren]

Steps to reproduce:

1. On Dashboard Click Create New Article
2. On the Editor page, type in Topic Title: 31Dec2020 and Sub Topic Title: 31Dec2020s
3. Click on save draft.
4. Click on Back button
5. On Dashboard Click Create New Article
6. On the Editor page Type in Topic Title: 1Jan2021 and Sub Topic Title: 31Dec2020s
7. Click on save draft
8. Observe that an error message is shown saying 'Topic title' already exists.