Reported here https://github.com/bitcoin/bitcoin/pull/12629#issuecomment-381095603 by @Empact.  I've seen it now in a few different travis jobs, including:

https://travis-ci.org/bitcoin/bitcoin/jobs/366157920
https://travis-ci.org/bitcoin/bitcoin/jobs/366139851

From my initial investigation it seems like there may be an actual bug in block announcement, so I'm  opening this issue to track.
{
                    for (const CTxIn &_txin : ptxConflicting->vin)
                    {
                        if (_txin.nSequence <= MAX_BIP125_RBF_SEQUENCE)
                        {
                            fReplacementOptOut = false;
                            break;
                        }
                    }
                }
                if (fReplacementOptOut) {
                    return state.Invalid(false, REJECT_DUPLICATE, "txn-mempool-conflict");
                }
                setConflicts.insert(ptxConflicting->GetHash());
            }
        }
    }
    {
        CCoinsView dummy;
        CCoinsViewCache view(&dummy);
        LockPoints lp;
        CCoinsViewMemPool viewMemPool(pcoinsTip.get(), pool);
        view.SetBackend(viewMemPool);
        
        for (const CTxIn txin : tx.vin) {
            if (!pcoinsTip->HaveCoinInCache(txin.prevout)) {
                coins_to_uncache.push_back(txin.prevout);
            }
            if (!view.HaveCoin(txin.prevout)) {