ANSWER:
If this is happening every prayer, you must ignore it as these are whispers of satan trying to destroy your prayers.
But if this happens once every blue moon, you must try to figure out whether you offered one or two. If you are inclined to either opinion, you must follow it and then offer 2 prostrations of forgetfulness after salam and then offer another salam.
However, if you were totally in the dark and don’t know whether you did one or two, in this case you assume you offered only one, offer the second one and then continue with your prayer as usual and then after tashahud, offer two prostrations of forgetfulness and then salam.