If a completion occurs after blk_mq_rq_timed_out() has reset rq->aborted_gstate and the request is again in flight when the timeout expires then a request will be completed twice: a first time by the timeout handler and a second time when the regular completion occurs. Additionally, the blk-mq timeout handling code ignores completions that occur after blk_mq_check_expired() has been called and before blk_mq_rq_timed_out() has reset rq->aborted_gstate. If a block driver timeout handler always returns BLK_EH_RESET_TIMER then the result will be that the request never terminates.
OK, now I understand how we can complete twice. Israel, can you verify this patch solves your double completion problem? Given that it is, the change log of your patches should be modified to the original bug report it solves. Thread starts here: http://lists.infradead.org/pipermail/linux-nvme/2018-February/015848.html