Hi,
is not so simple...
I will do this:
First, count how many mailadres are in the table:
SELECT mailadres,count(*) as num_of_equals FROM table
GROUP BY mailadres
ORDER BY num_of_equals
This query tells you if there are some mailadres with more than one entries.
Then for each mailadres where count()>1 do any action.
I hope this helps you.
bye bye