Extract Domain from email in Excel or Google sheet

 =TEXTAFTER(A2, "@")

or,

=MID(A1, FIND("@", A1) + 1, LEN(A1) - FIND("@", A1))

Google Sheet Sigle column No Duplicate Data Validation

 Here the example is given for column A:

=COUNTIF($A$2:$A711,$A13)<2