* Cyber-Stein's Effect: "You can discard this card to Special Summon 1 "Cyber" monster from your hand or Deck, ignoring its Summoning conditions."
* Doppelganger's Effect: "When this card is Normal Summoned or Special Summoned: You can target 1 monster on the field; this card's ATK becomes equal to that monster's ATK until the end of this turn."
The Issue:
* Cyber-Stein's effect doesn't "Normal Summon" or "Special Summon" Doppelganger. It directly Special Summons a "Cyber" monster from the hand or Deck. This means Doppelganger's trigger condition isn't met.
To activate Doppelganger's effect, you would need to:
* Normal Summon it.
* Special Summon it using another card's effect that specifically states it is a "Normal Summon" or "Special Summon".
In short, Cyber-Stein's effect doesn't involve a proper summoning, so Doppelganger's effect cannot be activated.