If you dig through contemporary pop music, you can find thousands of examples of songs that share a significant part of their overall chord structure. Sometimes the more recent songs were created on the patterns of earlier chord progressions unintentionally. At other times, I'm sure that chord progressions were mimicked completely on purpose. Some artists have even mimicked their own basic chord progressions in subsequent writing (Chuck Berry might be a good example).
If you know a bit of music and want a slightly more detailed overview, there's an interesting article on wikipedia that I came across the other day while doing some music assignments:
http://en.wikipedia.org/wiki/Chord_progression
You should note that this is not a very complex article, so you need very little music background to understand it.
But does this whole situation mean that we're running out of songs? Is it getting harder and harder for musicians to come up with original song titles, lyrics, and music? Well, yes for song titles, although it's gotten to the point now where many song titles have been recycled for perhaps dozens of songs. Finding something completely unique for a song title is pretty difficult.
As far as lyrics and music go, the situation isn't so bad. I've heard lots of key phrases in songs that seem very unique, but which are mirrored in other songs that appear to be completely unrelated. Maybe they ARE unrelated, and maybe they aren't. And chord progressions form the background of songs, but a lot of work can be done in production to make them sound entirely different.
I'm going to leave you with a bit of an interesting example of how chord progressions can be mimicked, but in a way that it does not seem like the "new" song is copying the old. I don't know if the mimicry was intentional in this particular case, but think about two songs: "Imagine," by John Lennon, and "Jump," by Van Halen. Then take a moment to check this out:
http://soundcloud.com/jackstanleywp/imagine-a-jump-john-lennon-vs-van-halen
I think you'll find it to be an interesting listen.
Imagine-a-jump-john-lennon-vs.-van-halen by jackstanleywp
Now, someone needs to put together John Lennon's vocals with an instrumental from Van Halen. It works both ways.
Before I leave, check this out too:
