Cowboy Casanova RIPS OFF Polly by Nirvana
 

The guitar part in Carrie Underwood's "Cowboy Casanova" is a total rip-off from Nirvana's song "Polly"

I can't post a link since I'm a new user but if you search "Cowboy Casanova Polly Nirvana" on YouTube there is a mesh comparing the two songs' guitar parts

And?

Nirvana were hardly the most original band to begin with.

So... what do you want, Kurt Cobain to come out of his grave and ring her neck?
Country is generic and does copy off of a lot of songs, if this is the first time you've noticed this sort of thing, then... well... you're pathetic.
[Jake's Meanness of the day]

Artists have been borrowing pieces from other artists throughout musical history.

i only mention country because that's what this guy is mentioning, but you're right it's been happening a long time.
i've even stolen an original chord from and iron & Wine song, but I'm using it differently so most people don't notice.
and since so many songs are made up of basic chords (G, C, A, Am, D, E, Em, F) there a bound to be a lot of songs that sound similar or "ripped off"