The current 3.3 version has Manage keywords section, which is however a bit limited.
It would be good to be able to choose all the entries in database and then manually rework keywords so that there are no duplicates. To that end a user needs to be able to:

1. select several keywords and then join them to one.
2. or select several keywords and do replace with and provide a new keyword.

Perhaps (2) is more convenient from the user perspective.