Handle tickets with same ID

[aesteve-rh]

Adding a ticket with same ID as a currently active ticket is not handled correctly. It just overwrites the labels from the previous ticket without any warning message. Also, it may lead to inconsistent data as we can find remains of the old ticket that are not overwritten in the new one, and end up with a mix of both tickets, which is wrong behaviour.

Ideally, we should log a warning, remove the old ticket, and store the new one.

[nirs]

Or simply fail if a ticket exists. Current clients never add a ticket twice.