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

app/reducers/transaction.js

@@ -105,7 +105,13 @@ const ACTION_HANDLERS = {
   [RECEIVE_TRANSACTIONS]: (state, { transactions }) => ({ ...state, transactionLoading: false, transactions }),
   [TRANSACTION_SUCCESSFULL]: state => ({ ...state, sendingTransaction: false }),
   [TRANSACTION_FAILED]: state => ({ ...state, sendingTransaction: false }),
-  [ADD_TRANSACTION]: (state, { transaction }) => ({ ...state, transactions: [transaction, ...state.transactions] })
+  [ADD_TRANSACTION]: (state, { transaction }) => (
+    // add the transaction only if we are not already aware of it
+    state.transactions.find(tx => (tx.tx_hash === transaction.tx_hash)) ? state : {
+      ...state,
+      transactions: [transaction, ...state.transactions]
+    }
+  )
 }
 
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