Aleth keeps track of which transactions it has sent out so that it doesn't send out the same transactions repeatedly. However, it never removes transactions from this list during normal operation (more on this below) so the list can grow unbounded.

The list is defined here: EthereumCapability::m_transactionsSent

There's a function which clears the sent transactions list (EthereumCapability::reset), but I don't think it's ever called in practice...it only appears to be called in Client::rewind() and Client::reopenChain(). Client::rewind() doesn't appear to be called anywhere and Client::reopenChain() appears to be a debugging function (based on the function definition comment).

We should ideally clear transactions from the list once they have been removed from the transaction queue.