Example methods and apparatus to de-duplicate impression information. An example method includes accessing a first set of cookies and a set of user identifiers, the cookies and user identifiers corresponding to devices accessing media via the Internet, identifying a pattern in the first set of cookies, obtaining impression information from a database proprietor, the impression information comprising a second set of cookies, identifying a subset of the second set of cookies that are associated with a same person based on the pattern, and associating impressions corresponding to the identified subset with the same person.