[WIP][FEATURE] added error message for nonunique tagname

__Tasks:__
- [x] done
- [ ] got feedback

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none

@michaelzangerle These error messages should definitely be not translated, they are for the developers. Please solve that in a different way.