Questions Regarding Salesforce Opportunity

I would like to ask whether it is possible to import Salesforce Opportunity data into a Custom Data Object (CDO) in Eloqua.

The reason for this question is that an Eloqua Contact can be associated with only one SFDC Contact ID or SFDC Opportunity at a time. However, as far as I understand, Salesforce allows multiple Leads, Contacts, and Opportunities to be created with the same email address.

Given this, I would like to know whether it is possible to retrieve data by comparing the Lead ID stored in Salesforce Opportunity as a key with the Lead ID stored in an Eloqua CDO.