Equipment. My wireless mouse is doing that nowadays, I think because it's pushing two years old. I find hitting the mouse like a television, opening up the battery compartment and blowing the circuitry like an NES game, as well as charging the batteries fully seems to help.
Using an older mouse? I googled the problem a while ago and it had to do with the mechanics in the mouse itself.
How old is the mouse? What make is it? ANY Microsoft Mouse I have had, or any cheaper mice I have had, the left button has started dieing after 3/4 months, it starts by SOMETIMES double clicking items..
I had this problem before... eventually the mouse began continually scrolling down so I couldn't do anything with most webpages... so I smashed the mouse and got a new one, problem solved.
If it always happens, I think there's a toggle in the options menu that enables or disables mous clicks acting as double click. If it only happens occasionally, yeah sounds like mouse problems.