I physically own a copy of the Painkiller Triple Dose pack which includes Black and Overdose. I installed my copy of Black (which is disc 1) and proceeded to play online with no problem.
A couple of months later I picked up a new gaming laptop and of course I had to install Painkiller. There isn't a key with the retail version of Black at all it seems, and when I try to go online now, it says CD key is in use and to please try to connect to the server again later.
Any idea why that would happen? Does it somehow lock it in and activate just once and not work again for a second install?
Sorry if this question doesn't apply to the GOG version as well, but I figured I would ask here since you all seem so knowledgeable.
Thanks in advance.

Have you tried uninstalling it from your old computer and then going online with it on your new computer? If thats possible it could point to an online activation. I bought my games when they first came out and they have a cd key for multiplayer so if black doesn't this maybe how they handled it.