It's not possible to add a topic who's name is the same as a prefix of an existing topic
how to reproduce:
- go to the project's settings
- type foo:barbaz into to "Topics" field
- press save
- type foo:bar into the "Topics" field next to the existing topic
- Gitlab will complete the topic and propose "foo:barbaz" as a topic
- however no "Add new topic" choice will appear
- clicking into another field to make the completion disappear in order to be able to save the topic as is will either:
- not be possible, because the "Save" button will still be disabled or
- will not have any effect when pressing the "Save" button, since Gitlab will not save the new topic