If you gain interest on your funds you will have to pay interest on those funds (I know, duh right?). When making a post tax withdrawal after 1986, you also must draw interest funds at the same time. That is where you will be taxed, so no double taxation.
This is why having a 401k post tax can be very complicated. It's also a big headache for companies to keep the allocation of post and pre tax 401k, straight and will often inspire an internal audit when a member smells something fishy. This is why many companies prefer to NEVER offer them.