Active Directory accounts with same email address

We have a user who has two Active Directory user accounts. One is called wleblanc and his second account is wleblanc1.

only wleblanc has an email account and wleblanc1 does not have an email account nor it is associated with a mailbox. When people email a security group, both accounts are members of it and that security group is associated with an email address. When someone sends an email to the group, we always get a bounce back for wleblanc1 account as there is no email associated with it. Can you please advise if you can have two active directory accounts associated to one mailbox?