\C-o firing twice in Emacs 26.3

Question

I usually add a keybinding C-o to call other-window (in addition to default C-x o), by adding

(global-unset-key "\C-o")
(global-set-key "\C-o" 'other-window)

in my .emacs. However, on one machine (Emacs 26.3, MS Windows 10) this event fires twice for every key press. I can see that it actually fires twice by doing (view-lossage).

This also happens if I bypass loading .emacs entirely and execute the global-set-key manually. If I manually re-bind C-o to any other event, for example M-: (global-set-key "\C-o" 'next-line) RET, that event fires twice. This does not happen for any other C- combinations that I've checked so far.

Any ideas what might be going on, and where I should look to fix this? Thanks!!