Funciona para mim se eu usar um inteiro real como índice:
$ tmux new-window -t 1 -n cmd1 -d sleep 60
$ tmux new-window -t 1 -n cmd1 -d sleep 60
create window failed: index in use: 1
Com um índice não inteiro, não funcionará:
$ tmux new-window -t cmd1 -n cmd1 -d sleep 60
can't find window i1
Cotação da página man (tmux 2.5-4):
new-window [-adkP] [-c start-directory] [-F format] [-n window-name] [-t target-window] [shell-command] (alias: neww) Create a new window. With -a, the new window is inserted at the next index up from the specified target-window, moving windows up if necessary, otherwise target-window is the new window location. If -d is given, the session does not make the new window the current window. target-window represents the window to be created; if the target already exists an error is shown, unless the -k flag is used, in which case it is destroyed.