I'm using a MBP late 2013 w/ macOS High Sierra. I currently use a Magic Mouse 2. I've been noticing for a while that when I click on something, for example a message in the Mail app, then click on a different message, 50-75% of the time, they both become selected. Sometimes I can click 3-4 different items and they will all be selected, other times it will change focus on the third item. This happens in various applications.