It should be noted that this error message is not generated by a Windows system program, but by the AVG antivirus program. Very unclear error message though.

Also, the apparent tautology, "the filename exists or it doesn't" is false (though I am sure you are aware of that). The syntax of a rename command is usually rename filename1 filename2. When the error message says, "duplicate file name or file not found," it means that filename2 already exists or filename1 does not exist.