We did a webinar recently on this, but it was meant for users of Qdabra's DBXL. Are you using DBXL or qRules?
The concept is generally the same. You need to perform a search based on (in this case anyway) the user who has opened the form. If there is a form in the library that "belongs" to the current user, then you can show them a message or a link asking them to open that existing file instead of creating a new one.
If you're using DBXL, it's all documented step by step. Let me know and I can send you the document.
Otherwise, the key is to have a data connection that queries your existing data and then you can have form logic to detect if any of the records that were returned matches the current user.