How to reproduce:
1. Open the user's attached project "BugTest.zip"
2. From the Project tab, open Assets/Scenes/SampleScene.unity
3. Enter Play Mode
4. In the Input Debugger (Window > Analysis > Input Debugger), expand the Users section until all users and their paired devices are visible
5. In the Game View, press the "Add Device" button
6. Repeat step 4

Expected results: a new gamepad Gamepad:/user0pad* is added to User #0, devices under the other users remain unchanged
Actual results: a new gamepad is assigned to User #0, but devices are reorganized and each user now has a Paired Device that doesn't belong to it (e.g. User #0 has Gamepad:/user2pad* assigned to it)

Reproducible with: 2019.4.24f1, 2020.3.4f1, 2021.1.3f1, 2021.2.0a12 (Input System 1.0.2, 1.1.0-preview.3)
Couldn't test with: 2018.4.34f1 as it requires outdated versions of Input System

Notes:
- The issue occurs in the Editor on both Windows and macOS