It isn’t always a case of redundancy, as in the “Reading” example I gave in my comment earlier. It’s an example, and there will are more situations that don’t represent redundancy.
Also, I interpret the recommendation against redundancy in the sense that later it might cause confusion because there would be multiple tags with the same name in the tag filter and in the autocompletion.
But the fact that the application allows multiple tags with the same name to be created, but then only presents one is very detrimental to the usability.
Two tags may have the same name, but they are not the same. Why does DEVONthink only present one?
I’m sure that under the hood, two tags with the same name are two different records identified by an integer or a UUID as the primary key. The fact that they happen to have the same name value should be irrelevant and not interfere with functionality.
In fact, in the cases where there is indeed redundancy, the fact that DEVONthink is doing this makes it less likely that the user will notice the existence of tags with the same name, and therefore, less likely he/she will take the steps to address it.