Thank you for the reply! I looked through the keybindings and the command is there and is not being overridden elsewhere. I also tried changing the keybinding, but that didn't change anything. Changing the command worked, however, and the keybinding was fine with e.g. "unindent". The command "duplicate_line", however, didn't work with Ctrl+Shift+D nor any other keybinding I tried to set for it.
I don't think I have any plugins installed. I do, however, have the theme set to "Soda Dark.sublime-theme". Changing that back to default, however, didn't help.
Great to know the "<" thing is a known bug/issue/phenomenon. Guess those two aren't related, then.
EDIT: Also, for whatever reason, I can't revert to fresh settings. If I remove the data folder and reinstall ST2, it just says it can't find some modules, even though the installation is new and should've (as far as I know) included those.