Xbox Game Pass 1 euro for 3 months?

If I get the 3 months Game Pass Ultimate for 1 euro and, say, redeem a card with 3 months GPU on the same day. Do I have 6 months of GPU?

No lol, do you think Microsoft is being kidnapped

You would have to look in the redemption conditions. Some codes can only be redeemed by first-time users. But what you could do in that case is to create a second account and then redeem the second code there (after the first code has expired).