Nope — Airtable doesn’t enforce uniqueness of keys, not can it be made to do so. There’s a Deduplication Block in beta test, but until that arrives (and maybe even afterwards, depending on your requirements), your best bet is to build your own duplicate detection routine in Airtable and alarm on attempts to enter an already-existing email.
You can find information on how to do so in this reply, which contains links to a document describing the process and example bases demonstrating the code. You’ll also find a link to an updated version of the base in this later reply. What you won’t find, however, are links to a similar writeup describing the updated system with corresponding example code — because I haven’t finished it, yet. (I started on it a few days ago — yes, horribly negligent — and hope to have it out before the end of next week. If you can wait a few days, that’s probably your best bet.)
