Marketo merges information to a matching email address as long as that data comes directly into Marketo.
In your case, if Marketo sees two SFDC Contacts, it will choose the most recently created Contact to merge the data with.
You will need to merge the two SFDC Contacts yourself either inside Marketo or in SFDC. There are automation tools for this from RingLead or DemandTools.
I still don't really understand what the identifier is. Is there a rule Marketo uses for merging contacts? For example, if they have the same e-mail then merge, or if they have the same phone number then merge?
And if two contacts are merged in Marketo, will they automatically be merged in SFDC and vice versa?
Marketo will automatically match on email address - meaning that it will prevent a new record from being created if the email already exists. Note that Marketo is case sensitive - so email@example.com and YourName@company.com will be 2 different email addresses, but are possible duplicates. In SFDC you can have the same email as a Lead Record and a Contact Record, and when synced they will be two different records in marketo. This might be a duplicate or it could be done or purpose and may depend on your lead generation strategy and/or business model.
In Marketo, SFDC Lead and Contact records are mapped to Leads (the Lead Object). In the Lead Database - you can see a default Smart List of Possible duplicates. For example you might want to identify modilfy duplicate SmartList to use email address, First Name and Last Name. This will help you to assess possible duplicates in the database.
For merging records, talk with your Salesforce Administrator to understand policies and preferences for merging records to clean up duplicates. In Marketo, you can see the possible duplicates and you can merge the records -- however there needs to be workflow rules and/or permission to recognize the duplicate merge in SFDC. Different organizations can have very different policies and access levels on merging or deleting SFDC records. I like to work with teams to develop a policy to convert SDFC Lead records to Contact records as soon as possible - so that my "Contact Records" can better represent the "audience of people" (customer and prospects) that I want to market to using different segmentation strategies. If you have significant issues with duplicates and want more complex logic for de-dup it consider a deduplication partner like RingLead.
Heidi, thank you for your answer.
Just to be clear, if two leads are merged in Marketo, they won't merge in Salesforce unless I create a workflow?
This depends on your Salesforce set up and permissions. My experience has been that you have to have rights/permissions or a workflow rule in place for the records to be merged on the Salesforce side. I hope that helps.
If you merge the records on Marketo, but don't merge them in Salesforce they will be re-synced back to Marketo. So you have to make sure both Marketo and Salesforce are aligned on rules for merging records. I don't know if you are the Salesforce Admin or if there is another person or team that you need to coordinate with to make this happen.