Reported here: https://github.com/bitcoin/bitcoin/pull/10244#issuecomment-378035305

Travis log here: https://travis-ci.org/bitcoin/bitcoin/jobs/361243719#L2810

Note the JSON exception:

```
test_framework.authproxy.JSONRPCException: The mempool was not loaded yet (-1) 
```

That exception was introduced here: https://github.com/bitcoin/bitcoin/commit/cb1e319fe9e198c9c5cf5236fe9af5a3d748b9e8#diff-a0c8f511d90e83aa9b5857e819ced344R1617

cc @jtimon 
if (pool.exists(hash)) {
        return state.Invalid(false, REJECT_DUPLICATE, "txn-already-in-mempool");
    }
    
    std::set<uint256> setConflicts;
    for (const CTxIn &txin : tx.vin)
    {
        const CTransaction* ptxConflicting = pool.GetConflictTx(txin.prevout);
        if (ptxConflicting) {
            if (!setConflicts.count(ptxConflicting->GetHash()))
            {
                
                
                
                
                
                
                
                
                
                
                
                
                bool fReplacementOptOut = true;
                if (fEnableReplacement)
                {
                    for (const CTxIn &_txin : ptxConflicting->vin)
                    {
                        if (_txin.nSequence <= MAX_BIP125_RBF_SEQUENCE)