Term bases can be opened in the Terminology Manager or the Translation Editor. So for term bases there are two interfaces you can use.
Within the interfaces you have some separate options, so feel free to check them out. While the Terminology Manager is made for actual terminology work, the work in the translation editor gives a the same interface and feature set as during translations.
An option to recognize duplicates in Wordbee is currently not available. You can search for terms that you suspect are duplicates in the search bar and check how many times they appear. As a workaround you can export an Excel file from the terminology data base and use Excels option to find duplicates to locate possible double entries in the export which you can then check and correct in the data base itself.
You can mark all terms with as many custom field information as you want. You just need to configure the custom fields first and can then assign them to the data base entries. The same applies to labels.