To create multiple Twitter accounts, you must use a separate email address for each account. Here is a fast and easy way to have all your Twitter accounts use the same email address using a Gmail account.
Simply use a dot (.) anywhere in the email user name and all emails that use that address will go into your original email account.
If your email is firstname.lastname@example.org, use the email address
Gmail does not recognize the dot (.) in the username, but Twitter does.
Photo credit: JoshSemans