Why is a second purchase claim invalid when the seller is the same and the buyer is two different people, if the evidence for the first purchase is dated, but the evidence for the second purchase is later in time?

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If one claimant provides evidence of purchase in Muharram and the other in Safar, the second purchase is deemed invalid. This is because establishing ownership for the first buyer prevents the seller from subsequently selling the already-owned item to the second buyer. In contrast, the situation where two claims are presented against the defendant (as the single buyer) is different, because establishing the claim of purchase from each individual seller does not invalidate the defendant's initial ownership by negating his ability to sell what he does not own, whereas the defendant cannot sell his own previously acquired property to himself later.