Resolution:
No matter how many durables you have, and whether they are offline or not, both server and db only keep one copy of the topic messages. but
we will have one ACK for each consumer.

If you have 100 durables on a topic and 99 of them have acked all their messages but the 100th one is offline, you will have all the messages saved for the 100th durable along with 99 acks for each message from the other 9 durables.  This can lead to large db files due to the large number of acks.