Require unique value per form

As far as I can tell, if I have, say, an email field and I want to require that it be an unique address, that is across all of the forms and entries. It would be extremely useful for it to be per form; that way I can keep last year's form entries around to compare with the new, etc. It also means if I have an entry form and a feedback form, the same person cannot fill out both if they require unique addresses. Or am I just missing an option somewhere?

Yes, earlier this year we duplicated our last year's entry form, gave it a new name, updated some fields, and put it online. (The old form was still in the system at that point.) The first few early adopters who filled out the new form discovered they could not enter with the same email as last year. We deleted the old form and the problem went away. Having said that, I haven't tried the completely separate feedback form, to see if that's true, but I don't think we require unique email addresses there. This year's entries will be done in July, I'll try another experiment at that point.