Minecraft for Windows Code was redeemed by someone else. How do I get another code

you need ask for who sell for you the Code

If you visit https://help.minecraft.net/hc/en-us/articles/360035130391-Minecraft-for-Windows-10 under Having issues
with your Minecraft for Windows 10 code?, if you have issues with your Minecraft for Windows 10 code, you will need to get in touch with Mojang for further assistance. You may submit a support ticket at https://help.minecraft.net/hc/en-us/requests/new

Hello LogGuy

I know this situation can be frustrating and we're here to help. I understand you're having trouble redeeming a code for Minecraft Windows 10 edition. Where did you acquire the code itself? Is there a chance that it has been redeemed to another account or
that someone else has already redeemed it? Let us know a bit more information about what you're experiencing to determine how to best assist you. Take a look at this support article for a bit more information about redeeming codes:

How to redeem gift cards and codes

I did that, and I redeemed the code on my alt. But it still didn't give me free Windows 10 Minecraft. Am I not doing it right?

If you redeemed the Code on your "alt", then Minecraft Windows 10 is bound to that account only.

In order to get Minecraft on your PC then, you would have to open the Store app on your Windows 10 device, and in the upper right, sign out of your "non-alt" account in the store app. Then sign in with your "alt" account, and you should find Windows 10 Minecraft
in your account's library. Simply download Minecraft from the library, and you should be able to play the game on your "non-alt" account as well.

But only your "alt" account that originally redeemed the code can re-download Minecraft from the store. No one else can.

I am not an expert, and I have the same problem. I first bought the "Minecraft: Java Edition", I wanted to use the redeem code to get "Minecraft for Windows 10", so I clicked "Redeem", I got the code, but when I entered it, it said "The code has already
been redeemed.", I checked my order history, it said that I had bought "Minecraft for Windows 10" using the code, but when I clicked install, it keeps showing me a small window with my code. I then checked whether if the account/email on Mojang, Minecraft.net,
and Microsoft Store, it was the same. So I don't get why I can't install "Minecraft for Window 10".