If duplicate leads are an issue, be aware of how Marketo determines which lead is active

[Nicholas_Manojl]

well, I haven't really touched the API much in recent times. But my understanding was the leadID was on the way out and the GUID was on the way in. But I think broadly speaking you make some really excellent points, so I definitely support you on that.

[SanfordWhiteman]

marketoGUID is for Activities.

[Dan_Stevens_]

Yeah, here's the thread where we discussed that change about a year ago:

Re: An Important Change is Coming to Marketo's REST and SOAP APIs

...and the link to the formal update on the developer site:

http://developers.marketo.com/blog/important-change-activity-records-marketo-apis/