insert into where not exists - how duplicate key error possible?

why would this SQL fail with a duplicate key error. I am selecting rows to
insert which do not exist in the "insert into" table.

insert into prugdim ( collNum, dimensions, isNewPrice )
select a.collNum, a.dimensions, a.isNewPrice
from prugload a
where not exists ( select b.collNum
from prugdim b
where a.collNum = b.collNum
and a.dimensions = b.dimensions )

PRUGDIM has primary key of (COLLNUM, DIMENSIONS ). I am inserting COLLNUM
and DIMENSIONS into PRUGDIM from PRUGLOAD where COLLNUM and DIMENSIONS do
not yet exist in PRUGDIM.

I clear the PRUGDIM table. Run the INSERT stmt. Get the message
"duplicate key value specified". Then I run the INSERT stmt a 2nd time, I
get the message "310 rows inserted".

delete from prugdim
385 rows deleted from PRUGDIM in COURI7.

insert into prugdim ( collNum, dimensions, isNewPrice )
select a.collNum, a.dimensions, a.isNewPrice
from prugload a
where not exists ( select b.collNum
from prugdim b
where a.collNum = b.collNum
and a.dimensions = b.dimensions )
Duplicate key value specified.

insert into prugdim ( collNum, dimensions, isNewPrice )
select a.collNum, a.dimensions, a.isNewPrice
from prugload a
where not exists ( select b.collNum
from prugdim b
where a.collNum = b.collNum
and a.dimensions = b.dimensions )
310 rows inserted in PRUGDIM