As someone who also uses emacs, I type C-a a so often that sometimes I do the same in emacs and it leaves an extra a. It's quite rare though.

As a vim user, this was my attitude about ctrl-b ctrl-b for pgup because tmux uses that. But I got used to it. Like many a screen convert to tmux, I used to map to ctrl-a, but I found myself actually getting pinky/wrist pain from using just my left hand. As a non-emacs user, I'm not okay with that :P So ctrl-b forces me to use two hands.

I remap the screen escape key to Shift-Ctrl-^:

  escape ^^^

C-a C-a definitely annoyed me at first, but I got used to it and now I don't mind it at all.

Yeah maddening! But you can remap it. I use this in my .screenrc to make it C-f:

escape ^Ff

C-f moves forward a character which is also useful. The easiest to type character that had the least impact I could find was to remap to C-j. Works well in both Dvorak and Qwerty layouts and that's the one emacs shortcut I use so infrequently that I can live with having to type it twice.

C-Space here

That’s my default as well, worked great, except on a machine with shared sessions, a test driver machine where several of us will login as the same user: some of those people come from windows machines where ctrl-space gets mapped to ctrl-@, which is NUL.

So we had to find another prefix for those machines; fortunately, tmux can have two.

This (space->@->NUL) is mentioned in the tmux docs, tbh.

C-o here. And C-x or C-p on a few rare nested screens in one particular screen session that I always have open. Those are identified by differently colored tabs in the hardstatus bar which is always visible.

C-t for me

I map mine to C^] which I find to be a good balance between easy to type and not conflicting with other tools

This is actually a very handy readline shortcut to search for a character. Useful when editing long command lines. Granted, I use Ctrl-Shift-] more often (searches backwards).