Display user defined errors

[Ling]

I've a problem here:
I have a quotation form & would like to set my form so
that it could display an error msg if the user tries to
insert an identical quotation no into the quotation table.
I'm aware of the CVerr() function but I need to first
check if the quotation table has the identical quotation
number, need'nt I?
How should I go about doing it? I do not want to set the
quotation no to an autonumber field...