1) 1st check your mouse profile under the control panel and make sure the settings look correct (especially the button configuration -- double click speed should be near center, and button configuration should be unselected).
2) If it is wireless, maybe battery going bad.
3) If it is wired and 1 has been verified, possibly the internal switch is bouncing. In that case get a new mouse.
(but 1st try another mouse before you throw it away, to verify it is indeed the mouse).