So, app A successfully MQGETs both COA messages. Fix the portion of thie DB process to tolerate 2 COAs. If your design asks for 2 COAs, you get 2 COAs.
That's the guess - I don't know for sure how ...
What messages arrive in the reply-to-queue?
-> COA messages which App A can't consume due to mismatch of correlation id e.g. App A can't correlate COAs with request messages
I o ...