Apparently, when a file is added to a sheet index, and then is copied as another file, that copied file retains the bit of information that it already belongs to a sheet index. I believe this should be cleared when I file is copied, and not carried over to subsequent files.

I often have a preliminary plan set with "Plan" sheets for early submittals. It is very common, for later, more involved submittals, to make copies of those initial plan sheets to create corresponding detail sheets. I may have a "general" plan, which is copied, and references adjusted, to make Drainage, Grading, Roadway, etc. sheets. Why would I go through the entire plan-making procedure a dozen times when I already have sheets with my border, and the sheet limits all laid out already? But if I try to add those later sheets to the sheet index, they are unavailable because they were made from a file that is already in the sheet index.

This creates a fair amount of extra work to "Remove from Sheet Index" for every sheet model for every other file.
Can we please just clear that bit of data on file copy?