All the extra turn spells say, At the beginning of that turns end step, you lose the game. No worries, though
Once the first triggered ability is on the stack, it won't trigger again, even if more damage is prevented and more incarnation counters are put onto it
Interestingly enough, the Gateway Shade still functions just fine as a traditional play black Shade
Actually, given the popularity of DuplicantDuplicant in blink decks like Brago, King EternalBrago, King Eternal or Aminatou, the FateshifterAminatou, the Fateshifter, it's clear this enters-the-battlefield effect is pretty appealing to a ton of players out there
Both these cards are in Portuguese so if you're having trouble making out the text, that's why