modeling – How to represent a duplicate data store in a data flow diagram (DFD) in the Yourdon DeMarco notation?

Yourdon/Demarco avoids duplication where possible. For example, processes are unique, and data-flow names are supposed to be unique.

They do not mention uniqueness for data-stores nor external entities, mostly because the same stores and entities may be used in different levelled diagrams.

And they don’t mention anything about duplicates in the same diagram. They carefully avoid such situation because duplicates symbols make it difficult to visually follow the flow. I found only a single case of duplicate store in their whole book (fig. 9): there is no graphical distinction between the two occurrences.

Conclusion: try to avoid duplicates in the same diagram. If you can’t avoid it, and want to draw attention of the reader on the special situation, use and asterisk exponent behind the store name, and add a legend for the asterisk somewhere.