I just tried (using an SQL Server table with no unique or primary key restrictions). I get the same error, but only with datastore dictionaries. The error also implies that it's the synonym catalogue that already exists.
It looks like we try to add the datastore dictionary twice for some reason. A bit weird that no one has run into this before, actually. We always use text file dictionaries (since they're a bit easier to work with), but of course this should work properly.
Renato, I'll add this issue to our bug tracker. I hope that you can live with the spurious error message until we've gotten it fixed. We'll ping you here when we have it fixed, then the next release after that will contain the fix.