1. Merging in SF, also results to merging in Marketo. And also the other way around.
2. Clean up SF first, afterwards Marketo.
3. SF deleted leads can be deleted from Marketo too (unless you have a specific reason of course).
4. Next clean Marketo leads without any activity (not even opened a single email and any webpage visit).
5. Next clean Marketo leads the are very old without activity since x years (e.g. 2 years).
6. Next go through the remaining leads and segment them in source (list import, api, and so on).
E.g. List imports with no recent activity (besides receiving emails) can be dumped to Excel.
7. Add some additional steps that are relevant for your lead process.
8. Merge contacts (SF) that also have a duplicate lead to the contact.
9. Merge the remaining duplicate leads.
This is great, thanks so much for the advice. It's always a challenge to know what the right process is, and this helps us out a lot.