Hi Jamie -
1) You said you have Lead A in SFDC. Is it also in Marketo? Is the new lead form a Marketo form that pushes into SFDC? Or is it one of those SFDC click-to-lead forms that creates leads directly into SDFC?
-If it is the former - and Lead A is in Both Marketo and SFDC, and the form is a Marketo form - it will be de-duped. Marketo is smart enough to recognize.
-If it is the latter - and Lead A is in Both Marketo and SFDC, and the form is a SDFC click-to-lead form - it depends on your SFDC rules. SFDC doesn't naturally de-dupe, but perhaps someone at your company as implemented a de-dupe 3rd party app or process.
2) Yes. Marketo views SFDC as it Boss. If there are dupes in SFDC, Marketo will be like "Okay, Boss. I'll create a dupe in here too."