JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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You can add a quantity property to your product and limit it to 2 and then enable the unique question option by going to Preferences and choosing which field do you want to setup as unique:
This way they only accept one submission with that value (it can be name, ID, ...)
You can try and clone this test form:
Please try to submit the same name 2 times.
Let us know if we can be of assistance.