Thank you all for your suggestions! I've come up with a sort of kludgy solution, but it works for my purposes:

1. On textbox.Valid(), call the method to seek duplicates.
2. If duplicates are found, set a flag in a form property.
3. In the next textbox's .GotFocus(), DODEFAULT() then look for the flag; if True, .Show() the other form.

-- Charlie