Hi, I'm proud user of first gen DA, now 11y old. It had double click issue after 6y, but it stops after few months. Its possible that it stops after firmware update and synapse driver.
Unfortunate now it started not register clicks at all. I will try to clean the switch.
If it won't help I definitely buy another DA :-)
I manage to still use the mouse, by clicking button little sideways. It wasn't very comfortable to use this way, but it works. Anyway, i took mouse apart to check switches. What was surprising that switches worked better when pressing them just with finger/nails, making nice audible click.
Problem happens to be the bottom part of buttons that presses switch, all of them have this very noticeable dent in middle. File them off flat and done! Working like new!
File the plunger part that is used to actuate the switch 'til it's flat (having its height measured beforehand) and stick some solid piece of plastic or whatever onto it. You're gonna like it, just have to fiddle with that piece's dimensions.