Switching applications with CMD+TAB launches an instance of the target app For a while now, every time that I try to switch an application using cmd+tab when I stay in the target application for more than a couple of milliseconds, when I lift my fingers, a new instance of the target app is launched.

I've been looking everywhere how to change this behavior, but it has been in vain. Originally I thought it was an emacs bug but then I noticed it also happened in chrome.
Could it be that you are pressing Option at the same time?

There's a little known feature that can be used to open new windows in (some) apps from the macOS app switcher.

1.Cmd+Tab to the app of your choice
 2. Keep Cmd held
 3. Press and release Option
 4. Release Tab  

https://lifehacker.com/384167/open-new-windows-via-command-tab