The TIFF specification says: "No data should be referenced from more than one place.TIFF readers and editors are under no obligation to detect this condition and handle it properly. This would not be a problem if TIFF files were read-only entities, but they are not. This warning covers both TIFF field value offsets and fields that are defined as offsets, such as StripOffsets."
The TIFF module doesn't currently check this, and some TIFF files cheat on this point, e.g., by using the same data storage for X and Y resolution if they're the same. Since this is a violation of the spec with regard to file structure, this should really be checked. We have a request for this check.