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Rule #0: Marketo considers the email address as the unique ID for what comes from you marketing campaigns, but uses the SFDC ID as a unique ID for records that come from SFDC.
Rule #1 (consequence of the Rule #0): every person record (leads or contacts) that are not deleted in Salesforce at the time you sync will be added to your Marketo instance. And in the future, if you create 2 records with the same email address in Salesforce, they will show up as duplicates in Marketo.
Rule #2: on the 1st sync, Marketo will not deplicate anything between the persons that are already in Marketo (from previous imports or web activities) and the persons coming from Salesforce. This will result in lots of duplicates in Marketo. You should seriously consider deduplicating these immediately after the sync or will will create for yourself a lot of problems on the long term
Rule #3: At any point in time after the initial sync, f you merge 2 leads, 2 contacts or a lead and a contact (through conversion) in Salesforce, they will also merge in Marketo automatically.
Rule #4: when creating a smart list to shoot a campaign, if the same email address is present in the smart list twice, Marketo will only send the email once. It has it's own rules to decide which one it is using. Dan Stevens wrote an excellent post here about this.
Thanks for clarifying,
As per Rule #2 you mentioned:
We have more than 0.1 Million data in Marketo and in Sales-force we have nearly 0.3million(it includes 0.1 million Marketo data). When the First sync occurs does that mean the 0.1 million data in Marketo will create additional 0.1 million duplicates from Salesforce sync?
Yes, and sorry for the very bad news.
With this kind of volume, you will need to invest into a one shot dedupe service. You can look into Marketo EasyMerge service or look into the launchpoint for dedupe products. Searchg the commuinity too, there are many threads about these solutions.