Regarding Customization

Hello everyone, i'm new to this group and also new to vantaage.I have
a few questions to ask regarding customization.

1. How do i put validate if i wanted to put a new field called
"Revision" in the Quotation module. Mean if a quotation is already
exists, the user can copy or revise that quotation and the "Revision"
will increase by it self.

2. How do i validate a new field if that record already exists? I use
the AfterFieldChange but it will only validate once, if the user clear
that field and re-enter the same details, it will also save.

Thank in advance

Best Regards