What Problems Are Caused By Data Redundancy?

Data redundancy is a data organization issue that allows the unnecessary
  duplication of data within your Microsoft Access database. A change
  or modification, to redundant data, requires that you make changes
  to multiple fields of a database. While this is the expected behaviour
  for flat file database designs and spreadsheets, it defeats the purpose
  of relational database designs. The data relationships, inherent in
  a relational database, should allow you to maintain a single data
  field, at one location, and make the database’s relational model
  responsible to port any changes, to that data field, across the database.
  Redundant data wastes valuable space and creates troubling database
  maintenance problems.