I have the self-hosted version and did some tests, afterwards cleaned the database manually. I reseted all counters, auto-increments etc. Now I have the following problem:
If I send an invoice to a customer, the systems sends this email not only to him but also to a second customer. For example: Customer with database ID 26 should get the invoice, but customer with ID 4 also get’s it. The next invoice for ID 27 is also delivered to 5, and so on. This behavior is even shown in the activity protocol, every email appears there two times but both times with the name of the right customer.
It appears that something I forgot to clean in the database, but even afters hours of looking I could not find it.