well a lot of the bands with buzz around them lately, ie. The Strokes and The White Stripes aren't exactly original but it is still damn fine music.
Most similarities in music I think are coincidences or tunes half remembered. I mean out of all the music people hear someone is bound to come up with a tune that they do think is their's but really they are remembering it, even then chances are it will have a different take to it anyway. God knows I worry everytime I write something and think hey that is cool, hmm, must be someone else's that I am remembering.

some of the best music i've written / been involved with has started as a 'how-the-hell-did-they-do-that?' exercise - you explore and understand that stuff you like/admire, and then, sooner or later, the lights pours out of you and somehow your own twisted slant on the whole thing becomes something beautifully unique.

We are but our influences, but always our own representation of the many tangled voices that speak within us ....

Originality in music? Hmmmm
I personally believe that this is a basic postmodern argument, which revolves solely around the fact that there are no new ideas, only old ideas made better.
Heres an example:
The strokes album "is this it" could never have been realised without the former knowledge/love of early glam/punk/rock i.e. Iggy Pop/Stooges, Lou Reed, Television, NY Dolls, Bowie blah de blah blah.
Anyway these "influences" also could never have made there contributions to contemporary culture without the former work of early "punk" (punk: as in "attitude") bands/people like MC5 and John Sinclair who were major producers of "punk" "new wave" and "metal" way back in the 60’s way before these words were even assigned to help classify music.
Now I must continue on to the fact that even MC5 weren’t that original in that they were only building on and making "heavy" (with Hendrix) the blues which bands like The Who, Yardbirds, The Kinks and The Stones sold back to America in the early 60’s.
So now it comes back to the blues, a very simple and expressive arrangement which evolved from enslaved black America and its "contemporising" of traditional African music,
Oh yes "It started in Africa".
So in short yes The Strokes have produced a highly referenced album, their contribution is in no way innovative, just a collection of catchy tunes packaged with some good lookin New York boys and a few names dropped by just the right people.
Its sad and a bit pessimistic but there are no really new ideas. Try and think of something amazingly new and "fresh" and sure enough its been done/said before.
The fret board is finite.

[ http://www.twolaneblacktop.musicpage.com ]

Awsome question! I would challenge anyone to NOT find incredible amounts of influence durring the major musical eras and cross pollination, otherwise known as IMITATION. The illusion that we
are separate and distinct has always been torn assunder by the
miracle of musical dialog. A good comparison would be inventions
such as the airplane or the lightbulb. In two very different parts of
the world there was concurrent development, so similar as to
make obvious the Truth that ideas pass THROUGH us not FROM us...

Its always been about who has transmitted the idea first, as if
pulled from the wind. Interestingly the blues from the lower
delta were so designed that a transparency to identity was
established. Many of the tunes were just part of everyone's set!

Similar with classical, as the innovations of one led to some
from another who built upon the foundation layed by the former.

Gee, doesn't nature work that way too?

:)