Do you see leads in Marketo's "Possible Duplicates" System Smart List? If not, you can import using the email addresses as the unique key. We do not allow duplicates in Marketo and it is not a best practice.
Alternatively, you can create a smart list and use the Email Address filter with the email addresses from your list. You can scan that for duplicates and if it's OK, you can add the smart list to a static list if desired.