Gmail doesn’t recognize dots as characters within usernames (email). So when one register with a email like saiful@gmail.com He can register in your wordpress site using same email just by using some of Gmail’s Trick.
("+") sign and any combination of words or numbers after your email address. For example, if your name was saiful@gmail.com, you could send mail to saiful+islam@gmail.com or saiful+lol@gmail.com.("+") anywhere in your email address. Gmail doesn’t recognize periods as characters in addresses they just ignore them. For example, you could tell people your address was saiful@gmail.com, saiful.islam@gmail.com or s.a.i.f.u.l@gmail.com.I decided to solve this issue , so i wrote a plugin which prevent bad guys from using same mail for registration. I wish you guys enjoy this plugin.