How to make sure a form / entries are not sent multiple times by the same user?
I m repeating your yelp example and I would like to improve it by making sure the same person cannot enter the same entries multiple times.
Looking forward your answers 
You can add a database field like Sent: True or False.
Then when they send it the first time, update the field to True.
Add a condition that when user clicks submit, it should only send the form when Sent != True.
Just create a custom state on the submit button yes/no type. Then on the submit workflow at the first step add a “set state” to yes. Do the same on the last step setting this time no.
Go to conditions of the button and create a condition that says when this button “custom state name” is yes: This element isn’t clickable.
I suggest to add a reset inputs workflow after submitting.
If your question is to avoid same message multiple times maybe there’s a plugin for that.