unique rows

I want to make a table that has unique rows - no problem.
Create Table x (smarts Varchar Unique);

I have input data that has many duplicates and until now
I was creating a temp table allowing duplicates, and then
Insert into x (smarts) select distinct smarts from tmpx;
This is now failing when I have hundreds of millions of
rows, running out of temp space (while trying to sort to
accomplish the distinct?)

So, is there a way (a constraint, a check?) that will simply
REJECT a duplicate when I insert the raw data directly into x
rather than FAILING on an error as it does
with the unique constraint above?

Thanks,
TJ O'Donnell