You have every right to ask, if anything I've always encouraged it.
I don't intend on sending steam keys to itch.io users when I publish it there (because of the price thing).
However many have asked me this question before and I've always replied with: Yes if you email me your itch.io account name i'll gladly reply with a free key or two on steam, so you don't have to buy it again.
As much as I appreciate your support, I still would suggest paying 3$ only instead of 20$. Once you're happy and have enjoyed the game and still would like to support it please feel free.
Thank you very much!
Really hope you do enjoy the game, cheers!