CiviCRM Community Forums (archive)

CiviCRM does NOT require unique email addresses. You may need to change the default strict dedupe rule however depending on how data is being input (to prevent unintentional combining of records based on same email address).

Joomla and Drupal do required unique email for Users - but this isn't a problem unless the members need to have logins for some reason.

That's the other way around: it synchronises Joomla/Drupal users with contacts. You can have contacts that aren't users and that have the same email address than a user/contact

X+

The default STRICT rule (which is what is used for self-service pages like online contributions and event registrations) is solely based on email address.

The way "matching" works is that each element in your rule that matches adds the "weight" amount to a total. If that total is at or above the "threshold", then it's considering a match. I suspect you have the default strict rule in place based on the behavior below (and you also have "allow same email address for multiple participants"). This is NOT a recommended combination of options since it will results in the problem you've described.

EXAMPLE with your setup:
First Name: Weight = 1
Last Name: Weight = 1
Email: Weight = 1
Threshold = 2

Existing Contact: Dan Green dan@foo.com
Input: Jane Green dan@foo.com .... 1 pt for last name match + 1 pt for email match = 2, considered dupe and will link event reg to existing contact and overwrite value (first name will be Jane after form is processed).

HTH

If many of your constituents share email addresses, you might want to consider setting that rule so that a new contact is always created and then using a different rule in your backoffice to periodically find and merge duplicates. You can do this by setting the threshold higher than any possible combination of weights.