When using the Duplicate option to create variant dirs, sometimes you may find files not getting copied to where you expect (or not at all), or files mysteriously disappearing. These are usually because of a misconfiguration of some kind in the SConstruct/SConscript, but they can be tricky to debug. The --debug=duplicate option shows each time a variant file is unlinked and relinked from its source (or copied, depending on settings), and also shows a message for removing "stale" variant-dir files that no longer have a corresponding source file. It also prints a line for each target that's removed just before building, since that can also be mistaken for the same thing.