When creating an account, I get the message:
A Steam account already exists with that contact email address. If you have previously created a Steam account, you should use the existing account rather than creating a new one.
This message will only display if your email address is already associated with an existing Steam account.
Creating a new Steam account will not allow you to register games which have already been registered.
Once you have created a Steam account and registered your CD Key or purchased a game on Steam, that game is permanently tied to that particular account. You can not transfer the game to a different account once you have registered it.
If you are attempting to access games which you have already registered, please see the Resetting a Lost Password topic for instructions to reclaim your prior account.