Hi Ellie,
You can use the Skip new leads and updates option (if this list you have has no new records for the system) which does not impact existing lead records and does not log any activities. Selecting this mode will: skip new lead creation, skip lead field updates, and will skip activity logging. This mode is used to create quick, pre-filtered static lists of existing leads for use in your marketing activities.
If you believe there ARE new records on the list, you can simply just import only the new records into the system- knowing that they are not in Marketo or SFDC yet.
To do this, you can create a smart list with the "Email Address is" filter and copy and paste the email addresses in your list import into the query region. After you have entered all of the email addresses (you may have to use multiple "Email Address is " blocks as they usually only hold around 1,000 values) you can then click on the Lead tab. This will provide you with the list of individuals that are on your import list and are also already in Marketo/SFDC. From there, you can download this list and run a VLOOKUP in excel to compare this list from Marketo and your import list to attain the net new individuals to load into the system.
Let me know if you need clarification or more information.
Thanks!
Morgan