Marketing Form Causing Duplicates

I created a Marketo LP that houses a Marketo Form. The landing page will be used by Sales to route lead and contacts to fill out the form. The problem is that filling out the form is causing duplicates despite that lead/contact already existing in Marketo. Why is this and how I can resolve it. I have seen some test leads of mine get merged but I am worried about the disruption and manual check it is causing.