Yes, we realise that.
If the names are not that clear, looking up the marriages for the parents in all probability confirms their surnames, as the "Alexander Buchan married Ann Duthie 1854 at Peterhead" would seem to do so.
The marriage cert, should also show if the individual parents are "deceased" or not.
So what you end up with are time lines for previous marriages of the parents, and still living or deceased, in the case of the parents, taken from the marriage cert' that you posted, so either census time lines are used, or death dates are used to clarify who was married to whom, as in the second set of parents. The death cert's usually shows, "widow of, or widower of".
The only marriages that I have come up with so far for Alexander Cowie, between 1835 and 1870 is Ann Panton, Ann Cowie, Ann Pirie, Ann Lindsay and Ann Murray.
Regards
Malky