Hi, I've run into the same when creating a new window in tmux. This seems to be caused by /usr/share/fish/vendor_conf.d/00debian-profile.fish.
Perhaps it's better to use fish_add_path (which should prevent duplicates) instead of setting the whole PATH? Also, it might need revisiting after https://github.com/fish-shell/fish-shell/issues/5394 is solved. As a workaround, it's possible to touch $HOME/.config/fish/00debian-profile.fish . This prevents the vendor_conf.d file from being sourced. Best René
