mtilbury on "users with same email address"

I have several "users" registered on my blog that have the same email address but with periods in different places to show up differently? They are using different names but all the same variations of this email address

d.a.nn.ylike.melala@gmail.com

d.an.nyli.k.emelala@gmail.com

dan.n.yl.i.k.emelala@gmail.com

Is this some kind of a hack?

Thanks, for any ideas or help.