How to add a republished work to a pub. series where it is already contained with an old number

Every work can only be added once to a certain series. But a lot of books have been republished with a new series number.
For example: Dummheit ist gefährlich has been published as "Scherz Krimi 221" and later under the title Mord im Spiegel oder Dummheit ist gefährlich as "Scherz Krimi 957".
At the moment we can only add one of them to the series "Scherz Krimi".

Any ideas how to solve this problem?

The only way at the moment is to use "221,957" or "221 and 957" or something along these lines as the label for the book in the series (and probably add a note in the series description what the double numbers mean)

OK, thx...