If a file system is case sensitive, or a directory is case sensitive (which can be configured on NTFS), and you try to copy a file to a directory that already has two (or more) files with the same name that differ only in case, then you may receive this error. For example, you could have the file abc.txt and the file ABC.TXT
The only solution is to delete the excess files that are not required, or configure your profile not to ignore/skip the relevant files.