I have some duplicate email addresses within the database that have the same email. On occasion, there is no further information in any fields other than the email address. Each duplicate email address and it's own lead number, and it's own history.
All leads are uploaded to Marketo direct from CSV file.
Why then has Marketo created a new record for that email?
Marketo dedupes people entering the database based on form fill outs and list imports. Marketo will not dedupe records manually entered (New Lead button in the Database) or that come from your CRM.
For those that are dupes you can use the Original Source Type and Original Source Type filter in a list view to see how they got into Marketo and that may help determine where they came from.