How to reproduce:
1. Download the "ExampleFiles.zip"
2. Create a new project
3. Open the project's Assets directory using any file explorer
4. Extract the files from the downloaded archive to the Assets folder
5. Observe the Editor's behavior

Expected result: The files get imported without any errors and the Project window shows that there are two different files named "test.txt" and Test.TXT"
Actual result: An "Importing" window opens up and is counting up the interations, but never finishes importing or importing gets finished with errors in the Console and there are several fake duplicates (not present in the actual Assets directory) of the same file.

Reproducible with: 2019.3.0a1, 2019.3.10f1, 2020.1.0b6, 2020.2.0a7
Not reproducible with: 2017.4.39f1, 2018.4.21f1, 2019.2.21f1 (imports only one file)

Notes:
1. Tested on Linux (Ubuntu 18.04, KDE Neon 5.18 (Not supported)) and Windows 7
2. Windows doesn't allow to have multiple files with the same name, but different casing, because it's file naming is case-insensitive. Although, it is still possible to add these types of files from a Linux OS and later switching back to Windows and opening the project
3. Dragging files into the Editor does not reproduce the issue because one of the files gets renamed when importing
4. Testing was done on an NTFS filesystem, although the issue could also be reproduced on an ext4 filesystem (tested on Unity 2020.2.0a7 and 2020.1.0b4)