For no scientific reason but what my palm is telling me, I think that the best mouse on planet right now is the Logitech G9, small and extremely customizable (even in its size).-

In contrast to its price and greatness, my G9 started to misbehave. It registers two left-clicks in press of once. No, it is not the faulty drivers. I didn’t install them. It is the Omron microswitch to blame.

So, this mouse is in need of an operative treatment.

This glorious article is our guide http://www.overclockers.com/mouse-clicking-troubles-diy-repair/

Read it! and read it side by side to the Logitech G9 special handling:

1. Unplug the mouse from your computer USB port! Remove the mouse feet, under them are the screws
   
2. Unscrew and remove the upper cell. Beware; the upper cell is connected to the main controller with a flat copper cable. You have to press carefully the safety lock upwards to release the cable. In any case, do not just pull away.
   
3. 
4. Disassemble the microswitch
   
5. According to my "beware" note on the photo: pay attention not to bend straight this section of the copper foil. If you accidentally did that, bend it again to a 45 degree angle or there will be no clicking sound and no click reaction at all!
   
6. 
7. After you reassemble the Omron microswitch, you have to test it. So... plug it in and test it. Try not to short-circuit the board with your fingers. If it' s not working, unplug and goto 4.
8. If it' s working, you now have to connect the upper cell. Unplug USB. Keep the safety we talked about up (step 2), slide in the cable. Press the safety lock down.
9. Screw and place the mouse feet into position. Plug in. You are done!