My xbox app on my pc is wanting me to buy game pass for my account that already has game pass

Regarding Game pass on Xbox. If you have a game pass you can set your box as homebox.

Then, any other account can share your subscriptions, games, and apps on that Xbox. You can Buy Ultimate and then one of your sons when logged with your account on The PC can have a game pass. Then you must buy a game pass for PC to another son, but with
his account. If you want both of them to have their own PC game pass, then you must buy a game pass for each account, but with their accounts. Homebox

If i understand the problem, you want to buy them both an Xbox Game Pass subscription to play on their respective PC's ? Buy an Xbox Game Pass PC sub for each or a Ultimate Game Pass. Log their account on Xbox beta app on PC and buy them the correct sub.
Or use xbox.com with their account correctly sign in and buy them Xbox Game Pass for PC.

my father has a game pass ultimate. He's able to play pc/ultimate. my Xbox is home. When I try playing on the pc it asked me to buy it. how can you help me?

Are you logging in on your father's account, or your own account? Because my fiance wanted to play the Halo: MCC on the PC one time, but she had to be signed into my Xbox account in order to be able to play it because my account is Game Pass Ultimate. Otherwise,
you have to have your own Game Pass Ultimate subscription.

If my friend wanted to use my Game Pass on PC, they could easily employ the same workaround I've already described, the current implementation doesn't prevent sharing.

They should allow you to sign into the Xbox app with whatever account you want, and then limit the number of devices Game Pass can be logged into simultaneously.

Instead I'm stuck with their current implementation that prevents me from easily using it on my own PC.

So your saying my family of 5 with 1 pc has to have 5 separate game pass subscriptions to use game pass on there pc accounts? I don't need to do this with the Xbox why is it different on the pc? I should be able to buy game pass for the pc and everyone that uses it has access