What is the ruling on someone who swore not to do something then did it, forgetting the oath, and does his oath take effect if it was by divorce, and what is the meaning of "it takes effect only as a matter of fulfillment"?

The gist of the Fatwa:

For one who doubts whether an oath of divorce was an oath by Allah or an oath of divorce, the majority of scholars did not rule that the divorce legally occurred. Rather, some of them said that lots should be cast between the oaths, and some said that an expiation for an oath (kaffarat yamin) is required of him, and this is the more preponderant view, because swearing by emancipation or divorce is not legislated. And some of them said that he should refrain from his wife until the situation becomes clear.