Creating an alert when a duplicate is detected

Creating an alert when a duplicate is detected

I have a field called input.Client_PO i would like to create an alert when the user inputs a location that's already in the DB.

so fas i have

Client_PO -> On user input

If   (input.Client_PO == ***duplocate formula****);
(
      show alert_label;
}

of course the part i need help with is the formula for finding duplicates, once that happens the alert label will be unhidden.

thank you.