If I have Ancient Greenwarden on the field and I play Simic Growth Chamber, since Greenwarden's third ability isn't a may it means that Chambers ETB goes on the stack twice. My question is if I can choose the same land for both the ETBs and bounce only 1 land or do I have to choose a different land for each instance of the ability.
You will need to bounce two lands.
Simic Growth Chamber's ability does not target, so you choose the land to bounce as the ability resolves. Once one instance of the ability resolves, the land you chose to bounce is no longer on the battlefield. So, it is not an available choice when the other instance of the ability resolves, which means that you have to choose a different land and bounce it too.