"screen -R -D -S test" will create a session named test if it doesn't exist, or connect to it if it does
"screen -d -m -S test" will create a new detached session named test, whether it exists or not, possibly leading to multiple sessions named test:
There are several suitable screens on:
9705.test (06/18/2012 06:42:58 PM) (Detached)
9639.test (06/18/2012 06:42:57 PM) (Detached)
How can I create a detached session named test, but only if one doesn't already exist?