Goodreads Librarians Group discussion
Book & Author Page Issues
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Merging records with reviews
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If a user ends up with multiple reviews on a single edition, then yes, only one will be kept. The cache will show all of them for a few days, though.
I was going through the Adventure Time series where the individual issues had been added to the database; I was going to merge them into the trades as usual, but there are a lot of reviews on some of these.