Here is a list of Marketo's native de-dupe behavior by activity.

 

Activity

De-Dupe Behavior

Comments

List import

Marketo will match with the record which was most recently updated

Not cookied and fills out form

Marketo will match with the record which was most recently updated

Record will become cookied. Pre-fill and progressive profiling will work on the next form.

Cookied and fills out form

Marketo will match with the cookied record

Pre-fill and progressive profiling will work

Not cookied, sent email and fills out form

Marketo will match with the record which was most recently updated

Record will become cookied. Pre-fill and progressive profiling will work on the next form. Email and filled out form activity will be on the record sent the email.

Cookied, sent email and fills out form

Marketo will match with the cookied record

Pre-fill and progressive profiling will work. Email activity will be on the record sent the email. The filled out form activity will be on the cookied record.

Send email flow step in a Batch Campaign

The email will be sent to the record with the lowest Marketo ID, that is, the one that was created first in Marketo

Send email flow step in a Trigger Campaign

Emails will be sent to every record since Marketo handles each record separately

 

Additional resources.