MyChat Client settings, "Files", "Rename duplicate files when they are received without overwriting"
When you accept files to the folder that contains files with the sames names, the program will add the suffix _copyN (N is a copy number).
Why do you need this?
For example, you accept files with the same names in a different time. "Work report.xslx" or "Tax invoice.docx". The content of these files is different but the names are the same.
To save sensitive data, enable this option to rename duplicates, and you will have a full archive with all important documents (even if the sender saves them with the same names).