Don't want to hurt your feelings, man, as you've bravely put yourself out there with this screenshot ... but OMG. I'd really like to have a banner pop up when library manager has that many duplicates and/or missing objects that says: "Stop. Clean up this stuff before you proceed." There's no way to know if any of the duplicates are serious issues or are simply (as most must be) the exact same object being loaded multiple times without harm other than overhead. Too many duplicates to take the time to sort through them.
This is the library equivalent of the TV show "Hoarders, Buried Alive!". I've seen this more times than I can say when people have contracted me for help, so you're not alone - it's really common.
When libraries are properly managed there should be NO missing objects or duplicates.
To your question first, Jonathan, and then your screenshot. The migration libraries are indeed different from the original libraries. For any given release, the primary library (e.g. 17) can be linked along with any of the associated migration libraries without giving any library manager errors since the migration libraries have been stripped of any objects that they might have been in common. If you load the original library for, say 16, instead of the 16 migration library - you'll have a lot of duplicates.
If you grab the AC 9 library and link only the electrical folder ... then look at the Duplicate Objects list to see if there are any duplicates involving that folder. If not, you're golden. If so, then select the folder in lib manager and display the list of duplicates to see what's going on. You may find that you already have the content in another library. More likely, there will be a few elements that are not needed - and so you can either (a) clean up a copy of the AC 9 electrical folder and link THAT... or (b) embed the non-duplicate elements that you need from the AC 9 electrical folder into your project's embedded library...within a subfolder to keep track of where they came from.
Loading (linking) libraries has nothing to do with the integrity of your AC installation. The libraries are still wherever they were on the disk. You're just adding them to a particular project - either with good results, or with clutter (duplicates).
With your screenshot: a PLA contains ALL library parts that are used in a project at a minimum. If, when the PLA was saved, the checkbox was checked to save the entire library (not advised) - then the PLA contains the complete library for every libary linked into the original project. It also seems like you've loaded things both as embedded libraries and as linked libraries, further increasing the duplicates.
Take the time to whittle this down, slowly and step by step. For example, delete the link to "++ART WORK", click the reload libraries button, and determine what's missing - vs the 480 duplicates. See what is now newly listed as Missing... and decide how best to handle that - embedding if just a few images, or linking to a subfolder of ++ART WORK if more than a few and they reside in a common subfolder. Etc.
The fact that I see "Raumstempel" (the GER/AT room stamp I believe) and the Room Identifier INT in your missing objects 'from Attributes' suggests to me that something about your zone list came from somewhere else... and for some reason has some of the predefined zones associated with these non-US library objects. To clean up those 3 missing stamps - you need to look at Attribute Manager and replace them in your zone list with whatever stamp you typically use. Otherwise, there'll be just a missing object dot on the plan.
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