guests when completing a from are getting a message sorry only one entry is allowed

This is set to prevent multiple entries on the same form and normally occurs if the Form Settings has the Unique Submissions enabled as "Check cookies and IP".

If you want to disable that, Go to Settings > Form Settings > Show more options. Scroll down to Unique Submissions and select "No check".

Related Guide for reference: How-to-Set-Unique-Submissions-on-a-Form