Getting an error message after submission

what does it mean "multiple submissions"?

I checked this form and I can see that it is working properly. I was able to do a test submission twice. Are you still experiencing this problem?

The error message you're getting only appears if you set a Unique Submission or Unique Question in the Preferences. Please refer to these guides:
How-to-Set-Unique-Submissions-on-a-Form
How-to-set-Form-Limits-Based-on-a-Unique-Question

Hope that helps. Thank you.