the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
If a person doubts the number of times he has issued a divorce, he has to count only the number of times for which he is absolutely certain. This has been clearly stated by Ahmad b Hanbal, and it is also the opinion of Abû Hanîfah and al-Shâfi`î and of their respective schools of thought.
The reason for this has to do with certainty and doubt. Any presumed divorce that exceeds the number that the man is certain to have issued is something doubtful. The marriage, on the other hand, is something affirmed, so it cannot be invalidated by something that is doubtful. Consequently, a doubtful situation regarding the pronouncement of a divorce will not be upheld as a valid divorce.
On this basis, the ruling in this case will be the same as for the one who had divorced his wife less than three times. Thus, he is entitled to take his wife back and resume his marriage.
In other words, whoever is certain to have issued one pronouncement of divorce and is in doubt regarding a second pronouncement, for him a single valid divorce will be legally recognized. Whoever is certain of two pronouncements of divorce and is in doubt about a third, for him two valid divorces will be legally recognized.
And Allah knows best.
Source: Islam Today