What is the ruling of Islamic law concerning guarantors failing to pay the debt amount after swearing by Allah's Book that they had already repaid the amount before the due date? And is what occurred considered a legal oath?

The false oath taken by the two guarantors that they returned the money to its owner is a "yamin ghamus" (a perjurious oath), which is one of the gravest sins and a cause for entering Hellfire. They must repent to Allah for this heinous act, which combined a false oath with the squandering of rights. The borrower must also return the money to its owner. The oath alone is not sufficient to clear one's liability unless there is an upright witness in financial matters.