I think they should be patched out once you learn them, like in Oblivion.
It feels like a waste to see all of them in shops again and it confuse me because I sometime buy the same spells twice.
Unless you can teach them to your companions? Who knows? Would some programmer tell us why they didn't patched them out anyway?

Other than that, YAY MAGIC!

They should at least have a "Learned" marker in red or something. Or just make the text red.

Yes! Learned tag would be perfect.

Agreed, I wasted some cash on a tome I'd already learned!

You're not the only person in Skyrim. What about little Lars, he may want to take up Chain Lightning or summoning a Deamon.

Hmm I see the problem here, people forgetting what spells they have. Oh for a spell book of memory enhancement. Or maybe just check to see what spells you have already.....

Can you actually teach spells to party members?

It isn't that bad since the number of spells has been nerfed, I haven't have trouble since I played my first fully learned up mage for a while.