[DEV] Check that our code survives duplicate transaction in remote topic

Kafka provides "at least once" delivery guarantees so our code needs to be able to handle cases of duplicated transactions, which are possibly stand away in time from each other. We should check that such transactions won't ruin Lead and Reader communication.

Start by locating the Kafka remote-topic transaction handling and the Lead/Reader communication paths. Reproduce duplicate transactions that arrive separately in time, then verify that delayed duplicates do not disrupt communication; the issue names no files or tests, so the relevant entry points must first be identified.

Written by the indexing model from the issue text.