During the NYC ID account creation process, the error message "This email address is already in use" might occur if you have already created an NYC ID account using this email address at some point in the past.
If you do not remember the password you used in the past with this account, click on "Manage Account" on the CAPS Online homepage.
Then, click on "Forgot password" under the login button.
Enter your email address, and you'll be sent a link to reset your password for your NYC ID.