The error your getting when trying to submit data can be a couple items.
If you are updating a form, opening and editing information, you want to be able to resubmit thus overwriting the original form. In your data connection, the last screen where you would click finish, there is a check box asking you if you want to overwrite if that file already exists(something to the effect). You want to have that selected.
If you are filling out a new form, then it is because of the form name that is being submitted. You need to find a way to create unique names for the forms. Something like: Purchase Request-91108. That is just the name of what the form is with the date of the form is being submitted. The hold back on this would be that you could only submit 1 unique request a day. I mostly use combinations of the date, a random number and who is submitting the form.