After installation, I purchased the remaining songs from an album not realizing that iTunes was not correctly identifying the one song I had already downloaded.
After clicking BUY, the song already purchased was purchased and downloaded once again along with the rest of the album.
Did you purchase the "remaining songs" individually or did you purchase entire album as a single purchase?
I purchased the remaining, entire, album as a single purchase. The album also has songs that do not complete. They are cut off in the middle or somewhere before the ending.
The problem is not isolated to that one album, however. I just downloaded Abbey Road and I'm experiencing the same problem with some songs getting cut off before the ending.
This problem began to surface after I installed OS X Mountain Lion. Albums downloaded a few minutes before I installed the OS have not been affected.
I purchased the remaining, entire, album as a single purchase.
You did not purchase the remaining songs. You purchased the entire album (incuding that same song) as one purchase in addition to yoru previous purcahse of the single song.
If you purchase one song and want to purchase the remainder of an album with that song, see this
Redownload the purchases that are messed up.
See this -> http://support.apple.com/kb/HT2519