Can I withdraw a petition from USCIS?

Andrew asks, "Just to be absolutely sure that I would never be out of status, because I was not sure if my employment based permanent resident application will be approved, I also filed an I-130 and I485 for adjustment of status based on marriage to a US citizen (marriage happened after my I140 was filed by my employer).  It so happens that my green card has already been approved and I have received a stamp in my passport.  How do I withdraw my USCIS Forms I130 and I485?  Or do I even need to?"

Trust me, you will not get two green cards.  Since you used the same A# and all other identifying information being identical, as soon as the USCIS adjudicating officer starts to process your pending family based green card application, it will be apparent that you already are a green card holder.  In that case, your petition will be denied/rejected. 

The thing is that rejection/denial looks suspicious later on when you apply for naturalization and you will unnecessarily need to provide a lot of explanation on why you had a petition for green card when you were already a permanent resident.  At first glance, it might even reek of intention to commit some kind of immigration fraud.  So your best option is to write to the USCIS with a copy of your USCIS Form I797 (there is no set form or format for withdrawing your petition and you can simply mail a letter to the same address that the notice of action came from) and tell them you that you will like to withdraw your petition and close the case.  In a few weeks, expect to receive a letter to that effect from the USCIS, but just in case, keep copies of your letter.  In the future, whenever you do business again with the USCIS (for example, to sponsor a family member for a green card), be prepared to answer questions about this petition but simply add a note saying that you withdrew the petition.