Many of us deal with duplicate leads on a regular basis. Today, I just learned of something that's been causing issues for us, as we were always under the impression that Marketo considered the lead that was "updated last/most recent" as the active lead - and the lead that would be affected for things like list uploads, email sends, etc. That's not the case. This could have significant implications if you have a lot of duplicates in your instance. I just posted an idea that hopefully will get enough attention and change the way Marketo deals with duplicate leads: When dealing with duplicate leads, Marketo should stay consistent on which lead record is considered the active/current lead
NOTE: if you head over to the Idea page (When dealing with duplicate leads, Marketo should stay consistent on which lead record is considered the active/current lead ), you'll note that a serious flaw has been identified - specifically around the way Marketo re-uses old LeadIDs (when the original lead is purged from Marketo) and allows new leads to take on these old IDs. This could have serious implications, for example, for those of us that leverage the API in our environments.