Duplication of information is a something that needs to be carefully managed when producing calculations. There are times that some duplication helps, for example adding a definition of a value a second time might avoid looking back and forwards in calculations to find the original definition. Generally though you should avoid duplication.

The main reason for avoiding duplication is simple, calculations need to be checked and if you duplicate information, this has to be checked every time you include it. Sometimes this is not too painful, most engineers have committed to memory a reasonable selection of standard references and equations. But often this is not trivial requiring a look up to the original source. So normally, in any given set of calculations, you should only define something once and where you use it multiple times use the reference column to refer back to the original definition (e.g. the page number of the calculations where it was defined).

This principal can be extended further though. Usually on any piece of design, multiple sets of calculations might be involved. Where this occurs, it is often good practice to prepare a Basis of Design document or something similar. This document can include all common references and even derived information (such as geometric properties of a shape) so that all calculations have a single common source for information. Of course this means that the Basis of Design document will have to be checked with the same scrutiny as a set of calculations but at least it means that the information only has to be checked once.