The attached file is an example to reproduce the situation:

CM1 is started and a cache is filled up with some values. 2nd CM is now started and the same cache is filled there with values. As result I'd expect to have (in this case) 8 entries inside the cache store.
When using JDBC cache store and looking into the logs, for some of the key added in the 2nd node (even if they are unique) he executes UPDATE queries instead of inserts...

This only happens, when cache store is shared and transactions are enabled.