@Grégoire_Miche2   This may be a little too late, but you mentioned " The only that works well is merging a non sync'ed lead with a sync'ed lead in Marketo, as this causes no merge in SFDC." I tried manually merging two contacts (one sync'ed one not with the sync'ed one as the winner) and I get the following error: Failed to merge people [3896966]. Any thoughts on what is going on? Thank you. ... View more