System Information
Operating system: MacOS Mojave Version 10.14.6 iMac2017
Graphics card: Radeon Pro Vega 64 16GB

Blender Version
Broken: 3.0.0, f1cca3055776, master

Short description of error

When copying and pasting an instanced collection into a new blender file, you'll get the instance group AND the individual objects that comprise the instance pasted separately into the scene.

Example:

1. I have a collection that has two objects: Cube 1 and Cube 2.
2. I instance the collection and call it Group 1. I copy the Group 1 collection instance to paste into a new Blender file.
3. I paste Group 1 collection instance into a new Blender file, however it pastes the instance collection Group 1 AND it pastes Cube 1 and Cube 2 separately. There are now a total of 4 cubes in the new scene.

Prior versions of Blender allowed an instance collection to be copy/pasted and only the instance would appear in the new scene.