I have had that experience as well where Mkto does not de-dupe when there is the same email address. Not sure why that is the case, but it has happened. One way to check without importing is just to copy and paste all of the email addresses into a Smart List. If there are any duplicates, you can export the list and do a v-lookup within your .csv spreadsheet to remove the duplicates. Then, you can add the Leads that do exist (from the Smart List) to a Static List in case you want to update them in any way. Keeps things cleaner.
A VERY manual process, but worth it for big list imports in my opinion.