I have had multiple times that a buyer ordered my gig twice by mistake
(meaning, the buyer only wanted to order once, but bought twice by mistake).
Of course, I always agree to cancel the second order (mutual cancellation).
However, this cancellation influence my completion rate,
and this has already caused to lose my level 1 seller badge once.
A mistake made by the buyer (which you cannot control as a seller),
like ordering a gig twice instead of once by mistake,
should not influence your completion rate.
I contacted customer support, but they told me they cannot change my completion rate.
Hence, I think not all cancelled orders should influence your completion rate
and the level of your seller badge.