If there are duplicate records based on email address in SFDC both will appear in Marketo.
Marketo doesn’t de-dupe records coming from SFDC since we view the duplicates created in SFDC as intended behavior. When you merge SFDC lead or contacts records in SFDC, in general, the master record prevails, but in case of conflicts you can choose which values to keep.
After Merging of Leads or Contacts In SFDC
The default Lead Score field is added up. For example, merging 3 SFDC leads with scores of 10 each, will yield a result of 1 lead with a score of 30. This only applies to the standard Lead Score field. Custom score fields will not be added up. The custom score fields will retain the value of the master record unless you choose a value from a different record. One solution is to create another score field called “Total Lead Score” and not use the default Lead Score field.
The complete activity logs will be merged in Marketo including membership in all programs.
Even though the SFDC user chooses a Last Interesting Moment during the SFDC merge, a history of all Interesting Moments will appear in Sales Insight.
One way to identify merged records in Marketo is to use the filter "Lead Was Created" and add the constraint on Min. Number of Times = 2 within your timeframe.
Here is a different method to identify merged records by Jason Martin.
Lastly, here is a detailed article by Grant Booth on understanding merges in Marketo.
Where to Merge Records
Merging duplicates in Marketo might not be possible due to your SFDC profile settings for the Marketo user. If all SFDC fields are not synced to Marketo it is recommended to merge records in SFDC vs Marketo so any field conflicts will be addressed.