פירוש על עבודה זרה 108:7

The author of the other opinion now has to explain why he does not also use the word “it” to teach that only in cases of idolatry does the second exchanged object become prohibited but not in orlah and mixed seeds in a vineyard. The answer is that idolatry and Sabbatical year produce are two special cases, and only in them does the second exchanged object become prohibited. Whenever we have two verses that teach the same rule about two different cases, we don’t apply that rule elsewhere. The idea is that if the Torah wanted the rule to apply across the board it would have taught it once. By teaching it twice, the implication is that the rule applies there and nowhere else. Therefore, even without the word “it” we know that this rule does not apply to orlah and mixed seeds.