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- An institution assigns different catalog numbers to what is typically considered a single lot. Typically, this might happen if specimens from a lot are prepared differently. For example, some specimens from a jar of alcohol-preserved specimens are re-cataloged as skeletons and assigned a new catalog number.
- An institution splits a lot in order to donate or trade with another institution. The receiving institution typically assigns a new catalog number. This is often done as a safety net, insuring ensuring that if something happens to the specimens at one institution others will persist. This situation can sometimes be difficult to sort out since the two lots are subsequently managed separately and data changes at the institution level are typically not shared. A change in identification at one institution and not at the other can mask that these originally came from the same lot.
- Some institutions catalog single-specimen lots (each specimen is assigned its own catalog number). Typically, a database record (or museum lot) is defined as all those individuals of a single species (a jar of specimens) collected during a single collecting event, which is assigned a single catalog number. Assigning catalog numbers to individual specimens can greatly inflate a database with duplicates.
- Combining occurrences from various data sources (including museum specimens, state agency databases, primary literature) into a single database, which our projectnow project now does, can cause duplicates. In many cases researchers publishing articles or contributing to agency databases also place their specimens in museums.