The exact same data stored in more than one location in the same hard drive or database. Unnecessary redundancy can cause problems if one copy of the data is updated and another copy of the data is not. All redundant data can’t be eliminated in most databases because primary keys in one table are repeated in other tables as foreign keys to create links between tables. This type of redundancy is called controlled redundancy. Also - web hosting companies use power and bandwidth redundancy to ensure uptime.

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