Allows more than one with the same email address for membership. Why?
You shouldn't allow to membership of more than one person with the same e-mail address.
You should add a options to general settings.
Example options "Require Unique Email Addresses for each membership"
This has been discussed several times, and the general consensus has been that the current system is both convenient and appropriate. Several admins wish to have secondary accounts for testing, but only one email and many families share email addresses, but wish to have a separate account for reviewing genealogical data.
If you want a one-to-one match and prohibition to multiple accounts related to one email address, use webtrees as it prohibits non-unique email addresses.