How to reproduce:
1. Open the user's attached project
2. In the Project window select the 'Scenes' folder
3. In the top bar menu select 'BugRepro' > 'GetImporterOverride'
4. Observe the Console

Expected result: GetAvailableImporterTypes should return nothing for folders and should never include UnityEditor.AssetImporter in the returned array
Actual result: GetAvailableImporterTypes returns array of [UnityEditor.AssetImporter, UnityEditor.AssetImporter]

Reproducible with: 2020.2.7f1, 2020.3.8f1, 2021.1.7f1, 2021.2.0a16
Cannot test with: 2018.4.35f1 (PrecompiledAssemblyException), 2019.4.25f1 (due to script errors)

Notes:
- The issue is also reproducible with other types of Assets
- The logs return 'AvilableOverrides: UnityEditor.AssetImporter, UnityEditor.AssetImporter' or 'AvilableOverrides: UnityEditor.ModelImporter, UnityEditor.AssetImporter' depending on the Asset type