1999.10.29 04:18 "Re: Stupid TIFF question: tags in ascending order", by Tom Lane
Stupid question: does this mean strictly ascending order? In other words, is it legal to have the same tag twice (or more) in a single IFD?
AFAICS, the assumption is that a single IFD may have at most one instance of a particular tag. I can't find any text to that effect in the spec either. But it would obviously be bogus to have, say, two different Compression tags in one IFD. There is definitely no text saying "use the first one" or "use the last one", so I think a reader is entirely justified in rejecting such a file as incorrect.
There are some tags such as ImageDescription where you could possibly cope with multiple instances (say by concatenating the contents, in the case of ImageDescription). But in the absence of any explicit text in the spec saying that that's what should happen, I can't fault a reader for rejecting multiple instances of noncritical tags, either. I don't see any standard tags where a lot would be gained by allowing this, so on the whole the answer seems clear: no duplicate tags.
There are a *lot* of things that the TIFF 6.0 spec doesn't spell out as explicitly as I'd like. This is one. If Adobe ever gives up their TIFF-suffocation project, maybe there will be a new edition that's a little bit better written...
regards, tom lane