I completely agree with your point that using a supplier as a backup to itself doesn’t make much sense. The point of having a dual source is to ensure complete independence assuming there are any problems with one of them. There a multitude of events which can occur that would mean suppliers can’t fulfill their contracts. This includes natural disaster, work strikes, machinery fires, lack of resources and many others. This would lead to all product lines being affected in some way. This is why you must have entirely different vendors which can supply nearly the same exact product. They should be 100% interchangeable, different only in maybe price.