It usually comes from mis-numbering of works, published or not.  It can
occur in manuscripts, lists of a composer's works, etc.  There are two
such works in Szymanowski's output: the cantata "Demeter" op. 37 bis
(not published until 1970s when the edition of Sz's Complete Works was
started) and the song cycle "Slopiewnie" op. 46 bis.  Both cases are
strange, because: 1) Sz.  himself listed "Demeter" as op. 38 and the
next work, another cantata ("Agawe") as op.39 and there is no other work
to which op. 39 would be assigned; 2) similarly, there is no op. 47,
so there was no good reason to assign op. 46 bis to "Slopiewnie" (there
is no op. 45, either).

I don't know how "The Age of Gold" and its suite became op. 22 and op.
22a (there might have been another work with op. 23 before the suite was
compiled), but generally, when an "a" or "bis" is appended to an opus
number, there is no connection between the two - just as in the Kochel
catalogue (no connection either).  Shostakovich seems an exception here.

Since "bis" is a Latin word (meaning "twice"), it's used all over Europe.
BTW, when absolutely needed, the Latin "ter" ("thrice") is used - in
various contexts.

An off-topic digression which Dave may or may not let pass, since it's
not about music.  When I came to this country and heard people pronouncing
"x"" as "eks double prime" and "x"'" as "eks triple prime", I was
absolutely sure that it was a lame joke.  After a while I realized that it
was said in all seriousness, because the proper forms "eks bis" and "eks
ter" are simply unknown here.

-Margaret Mikulska