o que você quer é -R
do manual de tela :
Resume the first appropriate detached screen session. If successful, all other command-line options are ignored. If no detached session exists, start a new session using the specified options, just as if ‘-R’ had not been specified. This option is set by default if screen is run as a login-shell (actually screen uses ‘-xRR’ in that case). For combinations with the ‘-D’/‘-d’ option see there.