Find a way to prevent adding trackers from observed transactions

[lumtis]

The CI might currently add trackers for transactions that have already been added.

While it can be trivial to check a tx before adding the tracker. We should explore possible way to prevent duplicate at the protocol level.

Since an inbound is identified by tx + event index, we can consider adding an optional event index as part of the tracker, the tracker can't be added with the addInboundTracker message if this tx has been finalized already.