Index key prefix behaviour in MyISAM

[Hassan Bahrami]

I have a VARCHAR(510) column in my table and I'm gonna need a unique index (UNIQUE KEY) on it, since MyISAM index key prefix length limit is 1000 bytes I have to use index prefix like col_name(250).
My question is if this will lead to a duplication problem for the unique key or not?
it seems a posibility since this index will use the first 250 chars in the column which can be not unique although the whole column value is unique.