Should I log users in if they enter valid login info in registration form?

Recently, we've had users complain that they forget that they have an account, try registering, and get error message that the user with such email already exists. There is a proposal to just log them in such cases. So, if the user inputs valid login info into registration form, they are just logged in instead. Obviously, if the password isn't correct, user will not be logged in. What are the security implications of such approach? If the attacker already knows login and password, they will be able to log in normally anyway. Most sites don't have this behaviour, and my gut reaction is that this is not a good practice, but I can't articulate any specific objections.
http://dlvr.it/S0QkdZ