Adding duplicate vocabulary items result into unresponsive form

[jiru]

To Reproduce
Steps to reproduce the behavior:

1. Login
2. Go to /vocabulary/add
3. Add a vocabulary item, it appears underneath
4. Add the same vocabulary item in the same language
5. The loading spinner never stops. Manual page reload is needed to add other vocabulary items.

Expected behavior

5. The vocabulary item should get displayed underneath along with a message that says it was already added. Alternatively, an error should be displayed.