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If you try adding a line such as <dc:identifier>urn:isbn:1234567890123</dc:identifier> (inserting the ISBN-13), does it then work? What the dc:source is specifying is the ISBN for the source publication such as the print version. The ISBN for this ebook version must be different from that used for any other version.
In your case, the ISBN refers to the print edition of A Silent Voice, Vol 1.
Amazon does not supply ISBNs for it's ebook versions but you could try other sites such as Kobo which shows the ISBN for the epub version to be 9781632360625 though that is not included in the metadata within the Kobo supplied epub file.
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