If I increase the number of Rak'ahs in a Nafl prayer, should I recite the middle Tashahhud?

If you pray four rak'ahs with a single tasleem, you have the option to recite the tashahhud after the two rak'ahs (which is preferable), or to omit the first tashahhud and only recite the tashahhud at the very end of your prayer.

If you pray more than four rak'ahs as a voluntary prayer, it is permissible for you to recite the tashahhud after every two rak'ahs, or only at the very end of your prayer. Scholars have differed regarding the permissibility of reciting the tashahhud after every rak'ah, and An-Nawawi affirmed that it is impermissible.