The problem is that there is no constraint on the rsc_id, type and key in the identity table. The function inserting the entry does a sql select before inserting, trying to prevent this from happening. But the function is not in a transaction. Note that doing this in a transaction will also not prevent this from happening, because there is no constraint, so the database will happily add multiple entries.

Maybe just add a constraint and then match on the constraint error?

I'll first try to catch this in a unit-test. I was thinking about adding the constraint, and put the insertion itself in a transaction. I don't know how postgres will handle this.

It was possible to catch it in a unit-test. Added the constraint, which is working.

I will have to add code to update the schema of an existing database, but... there could be duplicate entries in the table. I'm a bit reluctant to automatically remove double entries.

Branch: https://github.com/zotonic/zotonic/tree/fix-3319