Robocopy with the /MIR option copies some files again and again every time it runs even though the files are the same. Obviously, robocopy thinks that the source file is newer.
How could it be? Only if the target file system has larger time stamp resolution. For example, NTFS can write the file date/time with accuracy of 100 ns. Ext3 has accuracy of 1 second only. Therefore, the file time stamp is rounded to 1 sec during the copy from NTFS to EXT3, which can automatically make a file on EXT3 older than the source file on NTFS.
The resolution is simple: robocopy has the /FFT option that uses FAT file time stamp. Accuracy for FAT time stamp is only 2 seconds.