Warn against duplicates

Hi,
I have an app for tracking returns and RMAs. While technically speaking we could receive the same part back more than once, this is a fairly rare occurrence, I was wondering if I can make a field check for repeated values (for example Serial Number), and if the SN already exists just warn the user that this SN already exist without making it invalid, just a warning, not a full on stop.

You could add a (possibly virtual) column of type Show that’s hidden until the condition occurs.