Why is my form not allowing multiple submissions?

We're having issues again with our players trying to register. Here is a copy of an email one of them sent:

"Unfortunately, when signing up I received an error saying "Sorry! Only one entry is allowed. Multiple submissions for this form are disabled". I was VERY careful to only push the button once. Would you mind seeing if my payment and form went through? Is it possible the form goes by date of birth? Both times I've gotten an error Matt signed up before me without an issue. Is it possible it thinks we're the same person?"

I found a similar thread that you posted here back in October:

https://www.jotform.com/answers/1271693

Since you never mentioned which form you're currently working on, I'll just assume it's still the same one that was reported on that thread:

https://www.jotform.com/72575461184158

That form BTW appears to be disabled - Users can't submit multiple submissions because you have set your form's UNIQUE SUBMISSION option to CHECK COOKIES AND IP. This would mean that if someone already submitted the form, they won't be able to submit it again on that same device.

To allow multiple submissions, please change it to NO CHECK.

Related guide: How-to-Set-Unique-Submissions-on-a-Form 

If you're reporting a different form, kindly share the Form URL so we can have another look.

Related guide: Where-to-Find-My-Form-URL 

So if it's a cloned form of a previous form, even though it has a
completely separate URL, it won't allow users to submit?
...

Yes, some settings such the submissions limit are carried over when you clone the form, other things like integrations will not be cloned.