Duplicate files opening from the Book utility
InDesign CC2025 v20.5 / Mac Sequoia 15.6
Opened all the documents from the Book utility then using the Book utility to navigate between them. Unlike previous versions, the CC2025 version is opening duplicates of the document instead of navigating to the one that is already open.
I checked the same file in the older version of InDesign, CC2024. It worked correctly.
I have been using InDesign for 17 years and have always opened or navigating to the documents from within the book utility. This is the first time in all these years that I've had this issue. I didn't realize this at first. You don't necessarily see it happening. But when I was closing the individual documents, I noticed that I had 4 times the number of documents open than I should have.
Note: My files are on my laptop, not the network. And I am the only person working on them.
Consulted the internet. It stated the following:
This behavior of opening duplicates or files as read-only when working with InDesign book files, particularly in collaborative settings, has been reported as a bug in recent InDesign versions, including CC 2025.
Here's a breakdown of the issue and potential workarounds:
The problem
InDesign book files are designed for collaborative work, allowing multiple users to access and edit different documents within the book. The software is supposed to show a lock icon next to a file if it's already open by another user, preventing others from opening it and making it read-only for them.
However, in versions like InDesign CC 2025 (and potentially 19.4), this functionality is reportedly broken. The software allows multiple users to open and save changes to the same file simultaneously, leading to the creation of duplicate files.
This issue also manifests as files opening as "unsaved" specifically when launched from the Book panel, even though they open correctly from other locations, according to the Adobe Community.

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Ivy Wood commented
Update:
Good news. The problem goes away after you save all the files to the new version of InDesign.When opening files that were created in an older version of ID, you will see the following at the end of the filename in the tab “…[Converted]"
However, the documents aren't truly converted until you save it. The first time you hit Save, it will actually be doing a Save As and you should save over the top of the existing file. The same thing will happen when you save the book file.
Now, after the book and documents within the book are saved from the new version of ID, it will behave like it did before. You can use the book file to navigate to each document and it won’t open duplicates. At least that is the experience I am having so far today.