Hi Nora,
Most reports and stats are built on the activity history of a lead. If you delete a lead you also delete its history so therefore the report will not be able to find the history to add to the report results.
For example, in a program you might currently have 50 members in various progression statutes, the numbers are calcualted by the leads that are currenlty members of the program. If you delete a lead you also delete a program member and therefore this will affect the numbers in your program results.
It's not an easy situation that you face, if you sync without deleting records in Marketo then you face having duplicates in Marketo, if you delete records in Marketo then you will loose valuable historical info on all the marketing that you have done to date.
If the number of potential duplicates is relatively small then you can set yourself the task of using the possible duplicates system smartlist and manually merging them yourself, keeping the SFDC record as the master. If you expect a large number of duplicates then I suggest that you look into the easy merge service that is available from Marketo professional services:
https://community.marketo.com/MarketoArticle?id=kA050000000Kyt6CAChttps://community.marketo.com/MarketoArticle?id=kA050000000KytBCASIf you're interested in Easy Merge, please contact Marketo Client Services for more information at
services@marketo.com.
There are also a number of appexhange dupe catcher packages available on the SFDC side. I can't personally recommend a particular product but if you were to go that route I think it would require creating a campaign that pushes all your Marketo leads to SFDC and then allow the de-dupe product to find the matching record in SFDC.
Regards,
Cathal