If you reall want to keep only those records where the email is unique (and remove all records that have the same email for the second/third etc time).
The most common method is to create a new table that has exactly the same format as the current column, but with a UNIQUE key (constraint) on the emails column.
Then simply copy all the records from the old table to the new table:
INSERT INTO new_table SELECT * FROM old_table
MySQL will give a warning/error for every record that does not have a unique email, and it will not insert those records.
Afterwards you can drop the old table and rename the new table to the name of the old table.
