Copying and Pasting a file in the same directory should either allow an overwrite of the original file OR paste a copy of the file with a slightly modified name (e.g. appended with 'COPY').
Copying and Pasting a file in the same directory results in a dialogue box asking if you want to Overwrite, but the Overwrite button is greyed out and unselectable.
It is common for people to want to make duplicates of the same file within the same directory for many reasons. Most file systems just make the copy without prompting for an overwrite, and modify the name of the file automatically so as not to create a duplicate.
Device: Pixel 2 XL
OS: Android 8.1.0
App Version: 3.2.1