Virginia Knight wondered why Beethoven's Op 81 contains two works of
different kinds written years apart and asked:

>So what happened?  Did Beethoven lose track of where he was and assign
>the same opus number to two separate pieces, leaving a headache for
>cataloguers?  Did the publisher or publishers goof?

Grove says the Sextet Op 81b was possibly written in 1795 and published
in Bonn in 1810, the Les Adieux sonata Op 81a in Leipzig and London in
1811.  My assumption from this is that someone, not necessarily Beethoven,
assigned the same opus number to two works.  To confuse the matter still
further, Beethoven's other Sextet, Op 71, was written in 1796 and also
published in 1810, but in Leipzig.

Richard Pennycuick