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As you can see in this video:

https://youtube.com/shorts/ASTPhRrFamI

The same object appears in 2 collections. I have 24 such objects and there is no way to get rid of one of the copy: if I delete one, the other one goes as well.
Changing the name to one also changes name to the other.
Can't move the objects into the same collection, and if I select one object in one collection, also the one in the other collection gets selected.
This suggests that have been created 2 references for 1 object in the COLLECTIONS list.

Made a test playing with NESTED collections, and apparently creating new collections and deleting them creates copies of the objects in the list, but they refer to the same objects in the scene (luckily).

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3 Answers 3

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This is not a bug.
You can use the same texture in several materials,
you can use the same material in several object data,
you can use the same objects in different collections,
you can use the same collection in different scenes.
Moreover, you can use the same material/object/collection in different files.
This concept of reusing data called linking.

In your example, you have object in two collections. If you move the object into a collection that's already having this object Blender supposes that you are doing something wrong and cancel the operation.

If you want to remove an object from a collection, select Unlink from context menu in Outliner, or remove link in properties:

enter image description here

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  • $\begingroup$ How do you duplicate the link into another collection? Shift? $\endgroup$
    – PolyMad
    Nov 28, 2023 at 3:23
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    $\begingroup$ @PolyMad there is an info about it in hint: Ctrl to link, Shift to parent $\endgroup$
    – Crantisz
    Nov 28, 2023 at 7:46
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You can put the same object in several collections, this is called linking, it can be useful to have an object being part of several collections, for example you want to use the collections to manage the visibility of a group of objects but you want this object to be visible in these different collections.

To link an object to a collection press ShiftM instead of M (M will only move the object from its current collection). To unlink press M and put the object in the wanted collection, or in the Outliner press RMB and Unlink.

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  • $\begingroup$ I don't think you saw the video. Why should I have two references in the COLLECTION window? I can't see a use for this. Also: this behavior is completely unexpected: I MOVE an object from one collection to another, I surely don't expect to have it duplicated. I suggest you to watch the video. Thank you for the info though. $\endgroup$
    – PolyMad
    Nov 25, 2023 at 9:41
  • $\begingroup$ I've just explained that you can have linked objects in different collections, it means they are the exact same object, an object can be part of several collections, it can be useful in some cases, the only thing that does not make sense in your video is that when you drag and drop from a collection to another it should disappear from the collection exactly like when I do a M $\endgroup$
    – moonboots
    Nov 25, 2023 at 9:48
  • $\begingroup$ Exactly, as explained: if I MOVE from one collection to another, I don't expect the objects to be duplicated, so I think 99% there is something odd going on here. $\endgroup$
    – PolyMad
    Nov 26, 2023 at 11:13
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    $\begingroup$ The inconsistency I've pointed out is very anecdotal, actually your original question was "there is no way to get rid of one of the copy: if I delete one, the other one goes as well", the linked objects are not copies, they are the same object that can be present in different collections, to remove from a collection right click and Unlink, the object will still be present in the other collections, and to delete for good in all collections, press X $\endgroup$
    – moonboots
    Nov 26, 2023 at 11:29
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    $\begingroup$ As I said for example you want to use the collections to manage the visibility of a group of objects but you want this object to be visible in these different collections $\endgroup$
    – moonboots
    Nov 28, 2023 at 6:12
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I solved by moving the problematic objects in a collection at the root level and then selecting DELETE HIERARCHY. This erased all the doubles inside that collection.

I have also found when this problem happens: if you have parented objects inside the COLLECTION list, and select them in there, and move them into another collection, then the objects will be duplicated to the new collection in the list, and the originals will remain. This really looks like a bug in Blender.

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    $\begingroup$ This is not a bug. You can use the same texture in several materials, you can use the same material in several objects, you can use the same objects in different collections, you can use the same collection in different scenes. You can also use the same material/object/collection in different files. This concept of reusing data called linking $\endgroup$
    – Crantisz
    Nov 25, 2023 at 7:46
  • $\begingroup$ @Crantisz I don't think you saw the video. Why should I have two references in the COLLECTION window? I can't see a use for this. Also: this behavior is completely unexpected: I MOVE an object from one collection to another, I surely don't expect to have it duplicated. I suggest you to watch the video. $\endgroup$
    – PolyMad
    Nov 25, 2023 at 9:40
  • $\begingroup$ @PolyMad In my answer I've just cited a case where you could be interested to have the same object in different collections, if you don't want the same object to be in different collections, just remove it (right click > Unlink), and to begin with, don't link it in several collections $\endgroup$
    – moonboots
    Nov 25, 2023 at 12:21
  • $\begingroup$ I didn't link anything in several collections. Not knowingly anyway, but I might have slipped a key hit unknowingly though. But again: this is not the same object in several collections. It is ONE object with SEVERAL REFERENCES in different collections. Meaning that it didn't matter which one of the 2 references I clicked, I was still selecting THE SAME ONE OBJECT ON THE SCENE. And I would like to know when this can come useful. $\endgroup$
    – PolyMad
    Nov 26, 2023 at 11:14

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