Here are a couple of instances where A R Rahman has reused some of his earlier pieces in songs. These are just 1 or 2 line pieces and there is no harm in doing so since the originals are also his. You can hover your mouse over my photo on the comments bar at the bottom of the track to find out details about the songs mixed.
I also read that one of the re-recording pieces in Roja was later the charanam of Telephone Manipol song. Also the rerecording piece of Kannathil Muthamittal became the pallavi of a song from the hindi film Ada. Other music directors have also done that. Harris Jayaraj has incorporated one rerecording piece from Vettaiyadu Vilayadu (plane sequence with Jo) in the song Mudhal Naal Indru from Unnale Unnale. Similarly HJ had used a rerecording piece from Minnale into a song, Pinju Thendrale, from Majnu.
What do you think of this? Also let me know if you know of any other similar reused pieces.