The LP (uppercut) part of HK, LP is the first move that activates DP (approximately 1/2 damage for subsequent hits). I am not 100% sure that all second hits of all combos activate DP because of a combo I did with Cyrax that had a bomb in the middle, unavoidable and comboed, and then the juggle after it did full damage. It is hard to tell if the bomb itself has something to do with the full damage however I know that if you do a grounded JK to someone that makes them hit a bomb, the rest of the combo will be DPed. However, it is certain that the final hit of any autocombo activates DP at the very least. Other combo hits like Liu Kang's pop up activate double damage protection (approximately 1/4 damage).
A combo like Jax's where you can leave off the last hit and do a sweep, LK or HK mid juggle from the autocombo shows that middle hits do activate DP at least for some characters. In a lot situations it's hard to test and would require Shang Morphs and Cyrax bombs as an example.
The teleport punch itself activates DP (1/2 Damage) without any hits before it, but it isn't special DP so it doesn't reduce the damage to 1/4 if done after something else that already caused DP.
An example of multiple DP properties in one combo -
Mileena:
grounded JK(DP), roll(DP), aaHPHP, JK, Air Sai(DP) 6 hits 39%
Three moves that activate DP but the first hit, the JK turns it on for the whole combo.
