How to reproduce:
1. Create a new project
2. Open the created project's folder
3. Open the Packages folder
4. Open the manifest.json file with a text editor
5. Add this line to the manifest.json file: "com.unity.ui": "1.0.0-preview.6",
6. Go back to the Editor
7. Observe the Console log

Expected result: No errors are thrown in the Console log
Actual result: "UnityException: The MakeClientFunc has already been registered. Registration denied." and "ArgumentException: An item with the same key has already been added." error messages are thrown in the Console log (Full error messages in the edit)

Reproducible with: 2020.2.0a18, 2020.2.0a20 (1.0.0-preview.6)
Not reproducible with: 2020.1.2f1, 2020.2.0a17 (1.0.0-preview.6)
Could not test with: 2018.4.26f1, 2019.4.8f1 (UI Toolkit not supported)

Note:
- The Package Manager won't open after adding the package in the manifest.json file and restarting the Editor, the following error message appears in the Console log: "NullReferenceException: Object reference not set to an instance of an object" (Full error message in the edit)