Musicians and composers copy ideas (from themselves as well as other composers) all the time; sometimes as a form of musical quoting, or perhaps as a restatement of a melodic idea. Personally, I think it's great.
Listen to the ending of The Beatles' "All You Need is Love." You'll hear the melody from the chorus of their earlier song "She Loves You." In fact, listen closely to the guitar line towards the outro of "She Loves You," and you'll notice it's the same line from "And I Love Her" [I don't recall which song came first, offhand].
I'm sure you're familiar with "Jump" by Van Halen. Take a listen to the guitar solo at the end. Sound familiar? Eddie Van Halen reused that same guitar riff in "Top of the World" from the For Unlawful Carnal Knowledge album.
Listen to Eric Clapton's solo in "Sunshine of Your Love" for an obvious reference to "Blue Moon" by the Marcels.
Think of "Hush" by Deep Purple. Doesn't the singing at the end sound strangely similar to that John Lennon line in "A Day in the Life"?
I was watching my DVDs of The Bob Newhart Show with my girlfriend one night. One episode had incidental music with references to the theme from Mary Tyler Moore. (Of course, it comes as no surprise since The Bob Newhart Show was produced by the MTM company). When I pointed it out that the incidental music was actually a nod to MTM, my girlfriend was like "Oh, so it is." 
Whew. Sorry for rambling on like that. It's just something that I'm passionate about, that's all.
