Error message: Sorry! Only one entry is allowed.

So testing my membership form and get this message on published form when trying it out

error message:  Sorry! Only one entry is allowed.

This is because your form has unique submissions enabled, to disable that you need to go to the settings tab, form settings section and click on the show more button: 

Scrolling a bit down will get you to the "Unique submission" option where you need to select "No Check": 

I hope this helps.