it's so confusing that the order changes when adding IDENTICAL strings to BOTH filenames. Is this really how it's supposed to be?
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I encounter this mostly with manga. (I'll not rehash what others have said).
FWIW, and in that use case, I deal with it by renaming x5 to x5.0 so it will sort before x5.5. And then usually put both into an x5 directory and then zip that into a CBZ.