'Unique Field Combination' trigger - prompt w/ warni

Upon filling on a new record form, I would like the system to alert the user that a combination of fields entered is not unique (example, combination FirstName and LastName). And, upon the alert, the user could decide to override the warning and create/update the record, or, after more investigation, cancel the create or update. The use case for this requirement is this... we have records representing people. We don't want two of the same person represented. The email field is the default check for a duplicate record. But, if a new record is submitted (that potentially represents a person already stored) with the same Firstname and Lastname but different Email, the form submitter should be alerted so he/she can view other metadata for the existing user and make a judgment if the new submission is in fact, a duplicate representation. With the "Unique Fields Combination" Data Validation trigger, it can only stop the data manipulation, not prompt the user with a warning that he/she can override or cancel.