פירוש על קידושין 92:12

The Talmud cites a source that refers to “one pot” and still the priest can keep the terumah even though the Israelite must separate terumah again. This contradicts what we said before about the Israelite not listening and again separating terumah if it is a case of one pot. In such cases, we should say that what he separated is not terumah otherwise he will come to think that he need not separate again.

In the case of separating terumah from bad produce to exempt good produce, this separation is valid terumah. The other produce can be eaten without separating more terumah, and the terumah he separated is real terumah. However, to prevent people from doing this, the rabbis made him separate terumah again. Thus in one container, even if the person does not separate terumah again, he will not be eating tevel (untithed produce). We can allow the terumah to remain with the priest without concern that a biblical violation will occur.