Advanced Search gives different dataset than same search in Reports
Advanced Search and Search Builder produce a list of Contacts. But a report can pull data other than Contacts and the Group By in the report can be such that you can get a Contact more than once in your output. For example when pulling a Membership or a Contribution Report.

For a Constituent Report if you're seeing the Contacts more than once: hit the checkbox for Contact ID (under Columns) - to confirm these are the same contacts in your CiviCRM database. If the Contact IDs are different - then go and merge them. If Contact IDs are the same then you are likely selecting a column that a contact has more than one bit of data for.