Duplication refers to the degree of replication of distinct observations per observation unit type. For example, in state-level secondary-education registration records, greater than 1 registration per student per official reporting period would represent duplication.

However, it is important to remember that while duplication can occur as a result of the accidental entering of the same information multiple times, duplication can occur many times as a direct result of the choice of level of aggregation; for example, aggregating to a single student registration per academic year when registration information is actually collected multiple times per academic year.