broker starts to raise "duplicate from store for queue://..." messages

In one customer environment the broker (which has been running for at least a day) started to raise thousands of warning of this kind:

2019-12-11 16:34:11,926 | WARN  | 2-WW] Task-17379 | Queue  | mq.broker.region.BaseDestination 878 | 162 - org.apache.activemq.activemq-osgi - 5.11.0.redhat-630377 | duplicate from store for queue://XXX, redirecting ID:localh0005f-152-1576070199-3:55:1:1:19303 for dlq processing

It is not clear yet what may have caused these warnings.

Some known facts:

• there is no network bridge involved here
• these warnings did not arise as part of a broker restart
• the problem was only seen on one queue
• message processing for that queue continued
• there where 100k messages produced to DLQ in less than 10 mins
• only a broker restart resolved the problem