Only ADD_TRANSACTION if we are not already tracking the transaction
Only ADD_TRANSACTION if we are not already tracking the transaction

LND’s SubscribeTransactions will announce transactions multiple times.
I haven’t looked into which circumstances this occur, but an easy fix
is to filter out the incoming transactions that we already have on record.

Fixes #317

LN-Zap/zap-desktop,LN-Zap/zap-desktop,LN-Zap/zap-desktop