Build ID W/Incrementing Digit

Help, I'm stuck! I have a Customer table:

ID Name
-- ---------------------
1234 Christopher's Surf Boards
4321 Christina's Nails

I have to build a Account table:
ID GPID
-- --------------------
1234 CHRIST00
4321 CHRIST01

I've built a function that gives me the six alphas and concatenates the
two digits. The GPID in the account table is the first six alphas of
the company name plus two digits to ensure uniqueness. (Don't ask why I
can't use the existing id -- the story is long and boring.) I built a
function that creates the alpha part and assigns the first 00. I built
a second function that I thought would check the Account table as I was
inserting and increment the numbers if the account number already
existed. It doesn't work. In the above table, both IDs come out as
CHRIST00. Here's the second function:

ALTER FUNCTION fnValidateID
(
@mString varchar(15)
)
RETURNS varchar(15)
AS
BEGIN
DECLARE
@sTemp varchar(15),
@sInc varchar(2),
@iInc int
BEGIN
set @sTemp = @mString
while exists (select GPID from dbo.tempID where GPID = @sTemp)
begin
set @iInc = convert(int,right(@mString,2)) + 1
set @sInc = convert(varchar(2),@iInc)
if len(@sInc) < 2 set @sInc = '0' + @sInc
set @sTemp = substring(@sTemp,1,len(@sTemp) - 2) + @sInc
end
--NEXT
END
RETURN @sTemp
END
My first guess is that the code is okay, so far as it goes, but that I
can't check the values in the table at the same time I'm inserting into
it. (The Account table is empty, initially.) In any case, I can't
figure out where I've gone wrong.
Thanks.