Users are unable to use the same email address for multiple accounts for security reasons. If a malicious party gained access to the user's email address and discovered their password, they would have access to multiple accounts in multiple SAM.gov UEIs, and could gain access to information from all of these vendors. Therefore, it is our policy that we do not allow multiple accounts to use the same email address.
As a workaround to this, we encourage users to contact their IT team regarding creating an email alias for use with FedConnect in this way.