Overwriting equivalent mempool transaction #4517

#4450 introduced a check for non-equivalent double spends and relaying, but for equivalent double spends it changed existing behaviour by still calling mempool.addUnchecked() on them. This breaks mempool consistency (detected using -checkmempool after a few days of running), which could be exploited to crash miner's nodes. #4484 seems to fix this.

With #4484, the IsEquivalent check occurs after boolean "respend" is set true. By that point, there is no way that tx can get added to the mempool.

This problem no longer exists as the change has been reverted.