Bollywood is a great one for formulae. The moment something -- story, actor, voice, location -- gets popular, they quickly clone it and serve it again and again till it is coming out of our ears.
The simplest version of this re-hash is a song in the same movie, which usually returns as a 'sad version'. The tune is great, the situation is ripe for some sadness, the audience is already in the theatre... what other reason do you want to hear the song once again? This is a practice which is getting slowly edged out because nowadays, albums have so many remixes, reprises and dubstep versions that happy and sad have become just too simple for us.
Here is a selection of alternate versions of very popular songs, including some (hopefully) unusual ones.
Mere saamne wali khidki mein (Padosan)
This is not a happy-sad split but an energetic-soothing split. After the manic energy of Kishore Kumar's singing and Sunil Dutt's lip-syncing of the original version, there was a 'romantic' version that was not there in the film (or in the original soundtrack) but included for the first time in a special RD Burman tribute album called Tumse Milke (released in 2000).
Tum bin jaoon kahan (Pyar Ka Mausam)
The makers just went bat-shit crazy with this gem of a number as it featured four times in the film. One happy-original version, sung by the father. One happy-reprise version, sung by the son to woo his lady love. One slow-but-not-really-sad version by the aged father, heard by the son on the roadside. And one sad-with-new-opening-lines by the son when he was forced to leave his lover. Phew!
Hum bewafa hargiz na the (Shalimar)
This is an interesting song where the happy version is almost unknown. While we have all seen Dharmendra singing the soulful version garnished with the exuberant tribal mating call -- Jhingalala Hu (x 3). At the end of the film, Dharmendra and Zeenat Aman got married in (what seemed like) a Christian ceremony that involved a maulvi and exchange of garlands, accompanied by the happy version.
Rote hue aate hain sab (Muqaddar ka Sikandar)
Sikandar (Amitabh Bachchan) had the penchant of singing morbid themes (embrace death because that's what you gonna get at the end) in happy tunes while zipping down Marine Drive on his motorbike. His friend (Vinod Khanna) remembered the thought and was assigned the tough task of singing the song in a suitably emotional manner when Sikandar popped it at the climax. As Sikandar always said, Zindagi toh bewafa hai...
Yeh dosti (Sholay)
Jai and Veeru were inseparable friends who sang the anthem of their friendship, riding on a (stolen) bike with a carrier, playing the mouth organ, fighting over a village belle and generally having a blast. When it was time for Jai to leave in the last scene, Veeru sobbed out the same song one last time to the accompaniment of crying violins.
Ek akela / do deewane is shaher mein (Gharonda)
I am quite undecided on which is the more popular version of this song. The happy version had a couple traipsing up and down the under-construction skyscrapers and talking about finding a home in the maze-like streets of the City of Dreams. When the dreams had come apart and only one half of the couple was left, he dragged his feet past the same skyscrapers and this time, the maze-like streets looked longer than life itself.
Dil dhoondta hai (Mausam)
The muted version of this song played with the film's opening credits as a millionaire returned to a small town that held many memories for him. As he explored the woods and paths of the hill station, he remembered the happy times his younger self had spent with a beautiful woman and the happy version of the song played, apt for the bounce he had in his step.
Yeh bandhan to sadiyon ka (Karan Arjun)
Brothers Karan Singh and Arjun Singh were chopped up by their evil uncle, but not before they ran around mustard fields singing a happy song about their undying love for each other and their mother. After their death, their mother harangued Ma Kali so much that the goddess sent back the brothers through a rebirth. As they reunited, a slower version of their 'theme song' played though this was actually a happy occasion.
SRK -- being the poster boy of romance and melodrama in the 2000s -- has sung a lot of happy numbers along with his heroines and after they separated, the songs returned in a sad version. Kal Ho Naa Ho, Jab Tak Hai Jaan all had 'double versions' of popular songs. But probably the most popular one is from the film that established him the darling of NRIs -- Kuch Kuch Hota Hai. The happy one was sung with his first love while the sad one happened when his second love was getting married (to someone else).