When importing data, Marketo should allow you to set criteria so as not to enter in data that could be in other regions or perhaps in another list.
For example, if you have a large lead list that you received from a tradeshow and one list is all of the attendees and another is just a list of attendees who visited your booth, Marketo should allow you to import in both sets of data without cleaning them first. You would enter in the booth visitors as a normal static list entitled 'Tradeshow123AttendedBooth' then uploade the full attendee list with criteria so that you don't add any of the booth visitors in the 'JustAttendedTradeshow123' list. When entering in the full attendee list you would code, Don't add lead to 'JustAttendedTradeshow123' List if included in 'Tradeshow123Attended'.
Does this make sense? Then I don't have to clean the data! YEAH!