Don't allow the same value for different custom field options

This for version 4.5.6: My overall goal is to set up a checkbox list of field trips for an event registration form. Hopefully searchable so we can get counts.
In setting up the custom data field I chose alphanumeric and checkbox, and enter 3 field trip options. (You have to chose alphanumeric to show the checkbox choice for some reason. The data should be boolean.) The end result on the form and in previews is that only the last checkbox option appears. Expected to see all of them. I was able to duplicate this on your wordpress based demo. You only need to preview the field set in the custom data set list to see this.