We think this usually comes down to a tired mouse with a dirty click.

If you select a message and then delete it using the Del or Delete key on the keyboard, rather than the mouse, does it still take two out at once?

So is the answer to this a replacement mouse, or is there a way to clean the mouse up.

Does the delete button work better?

Your mouse has microswitches - these are the things that click when you press a button. They have contacts and springs and can become worn, so that when they operate they exhibit "contact bounce" meaning that a single activation generates multiple switching events. Ideally electrical and software "debounce" measures should prevent this, but these are only effective up to a point and can't deal with extreme levels of bounce. Some of the filtering components that deal with bounce can degrade and fall in value, becoming less effective.

Microswitches are not repairable and it's unlikely that it would ever be worth the time and effort to replace them. So a new mouse is the fix for worn microswitches. But do you not have a spare mouse, or one you can borrow just to check the theory? If it's a laptop, do you get the same issue with the touchpad and its buttons?

Using the delete button works very well, I don't have a spare mouse, but as this mouse is eight or nine years old and an internet search has shown how inexpensive new mice can be I'm going to get a replacement mouse at the weekend.

I brought a new mouse on Friday and it appears to have cured the problem, I've had no sign of the problem with the new mouse.