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@Roy Klein Gebbinck: You misunderstand. This behaviour is correct and as intended. This is how hard references work. If you copy-paste an entity that hard-references another entity (in this case a line that hard-references a layer) the referenced entity should be pasted as well. One way to solve this is to save the dwg to a very old dwg format. To get rid of Xdata the Release 10 format can be used. Note that saving to this format also means losing other data. For example layouts are removed, dimensions become non-associative and hatches become anonymous blocks. Alternatively there are some purge applications 'out there' that might work for you.
@Miroslav Helebrandt: I found out today, DraftSight can delete this unwanted layers without problems.