Is there a way to import a list into Marketo that may have contacts that are already in SalesForce, without the list causing any problems with the individuals records in SalesForce?
Depends... what do you want to avoid?
I don't want it to overwrite the leads current information or history of interaction in SalesForce. If they are a hot lead I don't them all of sudden become a cold lead.
This shouldn't happen unless you are overwriting the specific fields that control these definitions, OR there is a smart campaign within Marketo that updates those values upon a change that you incite by importing.
Have you had this issue before when importing lists?
Understanding how you can block field updates is a good start: Block Updates to a Field - Marketo Docs - Product Documentation
Retrieving data ...