As a developer, I want to make sure the transaction pool never processes transactions it has seen before

[tizoc]

Currently our transaction pool, if it sees a transaction it has seen before (which can happen if it comes in a different message and doesn't get filtered out by the libp2p layer), it will process it again even if it was already in the pool, fix that.

[tizoc]

Update: #974 fixed it for transactions that already have been integrated in the pool, still need to take care of the ones that got already rejected before.