It happened when testing in a freshly installed beta client with no extensions or modifications, other than some changes in the general settings.
I can't rule out for sure that my mouse is bugging out in some fashion, but I've only noticed this behaviour in Opera so far, and nothing specific happened to my mouse recently, other than growing a few days older. Not sure if there's a way I can easily test this outside Opera and send feedback on that. It happens relatively often.
My mouse is a Razer Ouroboros, and I have Razer Synapse 1.18 installed. If you know of a simple, safe program that can test clicking/misclicking, I'll do some tests in that.
Edit: Tested for about 10 minutes on this site, clicking button 1, 2 and 3 in order. Had the mouse wheel in various positions. No double clicks registered. I'll try again later and post back here if I notice anything. Otherwise assuming it's not a hardware issue.