It is possible to place the same object in more than one collection. Select the desired objects in the Outliner, then drag them to another collection while holding Alt. This will add them to the new collection, as shown in Object Properties → Collections. From there, you can unlink the object from any collection of your choice.
Note: An object that is not in any collection is still part of the Scene Collection, so unlinking it from there can potentially delete it from the file. Also, to select the contents of a collection from the Outliner, double-click the collection.