I am just making a note for myself here and anyone else who might encounter this error.
I was trying to install TweetDeck in LinuxMint Gloria – a Ubuntu Jaunty 9.0.4 variant.
Could not be installed because another application is already being installed. (I didn’t grab the TweetDeck error screenshot so I reproduced it with Thwirl)
It just so happened that I had Synaptics Package Manager open installing other software and Adobe AIR installer doesn’t like it. As soon as I closed it, it worked like a charm.
I had looked around for solutions for this error without any luck. I hope you stumbled across this note without having to spend a ton of time in the interwebs.