Transactions are not to be added to the mempool if they already exist

[joelreymont]

We will receive a block from the network where transactions already exist in the mempool. Transactions are fungible so we will simply leave them in the mempool and not add them unless we can add the block to our current chain. #232 will delete transactions when we add a block.

[joelreymont]

@lucafavatella Make sure this is so and we'll close the ticket.

[joelreymont]

https://www.pivotaltracker.com/story/show/152481717