Nah it’s supposed to be undefined, just look up the actual source of you wanna check; chapter 5 is the last chapter in volume 1 and vol 2 hasn’t been released yet (so the standard procedure is to set it to undefined):
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Duplicate implies “copying” since it was uploaded/created by a different group of people which implies stealing/using someone’s else’s work
helpful correction (or whatever to call it. I didn't claim it
should be vol 1 and rather expressed confusion over it not being, so hard to call it a 'correction', but... 'explanation' maybe?)
Though I will need to correct a portion of the correction: duplicate does not imply stealing/using someone else's work, it implies that it is a duplicate of something. For a
manga chapter to be a duplicate, that would be when the chapter itself is a copy of another (in comparison, what you are referencing is when a
translation is a duplicate).
Normally that [having duplicate chapters] is not anything strange no matter how many groups post their own duplicate chapter (nothing prohibits ppl from inventing wheel more than once, particularly if it is because they wanted to put their own
spin (hehe) on the design), and it is only really worth (and ought to be) mentioning(-ed) when they are labeled to be different chapters (aka title_id or vol# or ch# is different).