An email address can only be registered once. If you are trying to sign up for a new account and discover you already have an account registered under your email address then you can use the forgotten password link to reset your password and access your account.
If you find the existing account for your email address is inactive: please see here
PLEASE NOTE: Springer.com (http://springer.com), SpringerLink (http://link.springer.com), AdisInsight (http://adis.springer.com) and SpringerMaterials (http://materials.springer.com) now share the same account login. So if you have previously signed up/registered an account in one site the same account can now be used to log into all the associated sites.