IdentifierEnroller plugin can be called multiple times using browser back button

An enterprising user has determined that during a self-service enrollment flow that has the IdentifierEnroller plugin attached as a wedge and configured to collect an identifier (UID), if the user enters a UID value and submits the form, then immediately hits the browser back button, the user can enter another value and submit the form again, and repeat ad nauseam. 

Longer term there should probably be some type of logic for enrollment flows that keeps the browser back button from "working", but in the meantime perhaps the IdentifierEnroller plugin could be updated so that if the CO Person already has an Identifier of the prescribed type then the plugin skips rending the form and continues.