I created a Form Builder form, with "accessible on front-end of website" ticked, "generic: allow all users" permission set; "contact source" = "log volunteer hours" on Individual 1; and "added by" = "Individual 1", "activity type" = "Volunteer", and "activity status" = "completed" on Activity 1. The security for both Individual 1 and Activity 1 is "form-based", and "allowed actions" include both create and update.
I'd like to use this form to submit a volunteer's hours (duration) as activity against existing contact. Right now, the form creates a new contact with the entered activity information every time it's submitted, but this isn't the right behaviour as it creates duplicate contacts (a different one for each activity logged). Have I got some of the Form Builder form parameters incorrect, which is causing this behaviour?