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There are three cases of duplicate records:

1. Two or more records have identical values in each and every field (true duplicates).
2. Two or more records have identical values in some fields, and the fields that do not have matching values are of no consequence (it does not matter which record we take).
3. Two or more records have identical values in some fields, and one of the variable fields gives us a vale we can select on (in practice, usually a datetime field like 'last updated time').