Chat log repeated with duplicate messages after reconnection to Matrix

[bkil]

The Matrix home server is slow and/or failing calls sometimes. There are many outages nowadays. Other legs of the bridge such as XMPP and Delta.Chat see the last few messages coming in from Matrix repeated every once in a while, sometimes multiple times in succession.

Matterbridge does not seem to produce any error in its log.

Loosely related: #1866

[bkil]

Probably the following was the root cause, however, it could be nice to have related error checking in Matterbridge as well, because it did not cause visible issues for interactive Matrix clients:
element-hq/synapse#17215