Thanks for the question! As I have probably written here in the past, DOIs are really citation identifiers. Content in a journal would be cited differently than content in a book - even if the content itself is identical. Because the DOI would be cited differently in the journal than in the book, the content should receive two distinct DOIs.
Then, best practice would be to link those two DOIs together using the isIdenticalTo relationship (assuming there was no new editing or review between the content in the journal versus what is published in the book) in the metadata record. More on relationships here: Relationships - Crossref
Because one is a journal article and the other is a book chapter (will this be a chapter in the book?), our system will not flag this as a duplicate, and the DOIs should not appear in your conflict report.