We use the SOAP API and have always set
ParamsSyncMultipleLeads smlParams = new ParamsSyncMultipleLeads();
smlParams.dedupEnabled = true;
smlParams.dedupEnabledSpecified = true;
on the parameters. Relatively recently, it seems like something changed and any sync lead calls will create new leads rather than dedupe them with existing leads that have similar addresses. In the past, the same code has returned "Updated" results as expected. Has anyone else noticed this or have there been any changes on Marketo's side that might account for this?