Finder sidebar duplicating entries
As I had backups of everything I decided to play fast &amp; loose with copies of the two .plist files.
I dropped both files into BBEdit &amp; tried to make sense of the reams of data.
The sidebar file looked at a glance more promising so I started with that.
I realised it had several thousand aliases to disk drives, SD Cards, CDs, RAMDisks &amp; disk images permanent &amp; temporary, dating back over perhaps a decade.

Carefully selecting sections from &lt;dict&gt; to &lt;/dict&gt; in each case, I deleted all aliases except the one that contained SeenAsBootVolume. Saved &amp; relogged.
Success. The current list rebuilt itself, the myriad 'spares' were gone &amp; the drive no longer duplicates if I move/drag items in the list.
As a hint as to just how many old drive aliases I did actually delete -  the file was 82MB… it is now 9KB.

 …and, no, the date modified on the older one doesn't make any sense to me either;)