[GraemeJ]

+1 for the warning when removing songs from the library.

........wouldn't a setlist specific note like “change key to A“ a sufficient workaround if you still don't like multiple song copies?

I've always thought the whole point of having the library was to maintain a record of every chart you have used, or likely to use.  Setlists are where you would normally display the chart for actual performance.  Charts themselves take up little space, so why delete anything from the library?

As for the transpose thing, I think the use of a note to denote a key change (or whatever) is a far better solution to this problem than asking Mike to write even more code (that might cause a problem elsewhere).