1999.10.29 03:27 "Re: Stupid TIFF question: tags in ascending order", by Daniel McCoy
I know of no language that explicitly states that any tag should appear only once in the file, but the fact that the spec does not specify what a reader should do in the case of duplicate tags is a strong indication that the authors intended it to be interpreted as strictly ascending order.
If you want to write tiff files that other programs have a chance of reading, you are much better off taking a conservative interpretation of points like this.
If you are thinking of writing files like this, it's a very bad idea.
If your intention is to store extra information that way, just get a private tag, you can make it variable length if you want and store pretty much anything you want in it.