פירוש על קידושין 92:13

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.