2007.02.01 09:42 "Re: [Tiff] Interpreting SubIFDs tag", by Gerben Vos
In fact, sometimes both are combined, and I've come accross files where the same second and third SubIFD gets pointed to in both ways.
It sounds like a bad idea to write such TIFFs. It's good that you have some reading code which deals with this in a smart way, but TIFF writers shouldn't expect readers in general to handle this. A TIFF copying program will typically duplicate the IFDs and image data that are pointed to multiple times, and this can grow rather fast as the number of paths to an IFD increases.