Fast DELETE with double subquery on big table for dupe remover
I wonder if that works at all, MySQL always had a stupid restriction: You cannot delete from a table and select from the same table in a subquery :-)

So create a temporary table first with the result of

select min(id)
from mytable
group by text; 


And use this in your delete

select from mytable
where id not in 
 ( select id from mytemptable);