1) Recording two vocal tracks for the same line in a song, making the line sound stronger. Not to be confused with harmonizing. Double tracking is one person recording two tracks to be played together. Harmonizing is two different people each recording the same track to be played together.
2) Putting two songs on the same track on an album. Most popular in the 70s to make an album have only 8 tracks (to fit on an 8-track), but have more than 8 songs.
1) John Lennon loved double tracking.
2) Some of the best examples of double tracking are Foreplay/Long Time on "Boston" and Jack the Stripper/Fairies Wear Boots on Paranoid.