I would suggest checking your submit data connection. In it, you are able to set the name of the form and check whether you want to allow overwrites. You need to make sure that the form is not changing names each time someone submits. The form therefore retains the same name and overwrites the previously existing form.
If you are allowing Save on your form, then you are basically allowing the users to input any name they want. You should only be allowing Submit to prevent user error and duplicate forms.
Good luck!