Doing this will potentially create duplicates. Instead - add the email addresses which you have in the SFDC report to Marketo smart list in the filter Email Address is, to see how much of your SFDC records actually exist in Marketo. The rest may be in SFDC and not in Marketo, or there could be more scenarios. However, if those records don't exist in Marketo - you would want to explore that why didn't they sync back instead of creating new records.
It's always possible. One quick way it to upload the data in Marketo without a field that is required for SFDC insertion. e.g. Company name.
Always upload in Marketo in a static list. Then use the "Person was created" filter with a "List" constraint to detect all the leads that were created during the import (vs. these that were merged).
But honestly, my first approach would be to troubleshoot and solve the discrepancies between the 2 systems, as fixing it a the core will go a longer way than the upload.