Currently, mongos will implicitly abort a transaction on ok:0 responses by sending abort to each participant shard unless two phase commit has been started. This means if a client sends abortTransaction explicitly, which also sends abortTransaction to each participant and waits for each to respond before returning, and any shard returns NoSuchTransaction, which may happen if the shard already aborted implicitly or explicitly, the router will re-send abort to each participant, which is unnecessary.