I tested 1.6 RC4 and RC5 with an order form where I duplicated a standard product field multiple times. Each has a slightly different name, price and description for each. The fields are not required, however, when I go to preview and test the form, the duplicated fields are highlighted in red and indicate that I need to "Please enter a valid value" when I select next to go to the next page (this is a multiple page form, the fields in question are on page 2 of a 3 page form). But even when I filled out the fields with a numerical value of 1 (there is no range) they are still highlighted in red and I cannot continue. To resolve the problem, I need to delete the duplicated fields and recreate the fields separately (not duplicate it).
This originally occurred in RC4. I upgraded to RC5 and was able to recreate the issue. I created a new product field. Duplicated it. Saved. Selected Preview and tried to proceed through the form leaving the non-required duplicated fields empty and got the same "Please enter a valid value" message. Entering a valid value does not help.
Tested in the latest version of Firefox as well and obtained the same results. I tried making it required, saving and then unchecking required, but no luck there. I disabled quantity, but still get prompted to enter a valid value when there is no place to enter a value.
I have been able to duplicate HTML sections without any problems.
I'm running WordPress 3.21.