Just create additional books and make note in title so it's not seen as a duplicate. For instance, retitle as follows:
Bad Moon Rising (Mult)
Bad Moon Rising (Mult)
Bad Moon Rising (Mult)
When you sort alphabetically, then you know that the book falls into multiple series so you don't accidentally delete as a duplicate. Use whatever code you will remember. I tried just one letter and then a year later, would forget what the heck that stood for.
Short memory!