Duplicate Contacts with same email address

Hi,

Can anyone advise of the rules regarding multiple Contact records existing in Dynamics which have the same email address? I was always under the impression that a Contact record could not be created if a Contact record already existed in the system with the same email address. 

I know duplicate detection rules can flag up duplicates with the same email address; however I always assumed there was something within Dynamics itself that prevented this happening.

I hope this makes sense? Any information will be appreciated.

Thanks,

Agnes