Yes, we have these kinds of sentences, so you can see that the same concepts can be expressed in different ways. I use these a lot in the Affixes lessons, because I want people to know that affixes can also be used on their own, adding to the language's flexibility.
In Esperanto they are completely right, and that's what this course teaches, so basically it doesn't really matter :)
Also, that sentence is the one I chose to use, but there are others, as Ludanto said, that are also right and not redundant.
but if A is not equal to A, then there's something majorly wrong with the universe.
Congratulations. According to the transitive property, you just defeated Muammar Qaddafi in arm wrestling.
What I like about these tautologies is than, even when they are absolutely uninformative from a purely semantic point of view, sometimes they may seem quite relevant and worth remembering from a more realistic point of view.
Ex-wives ≠ Ex-wife Plural ≠ singular
Your first sentence is plural (so saying some guy have 2 or more ex-wives).
Your second sentence is singular (so saying that the guy only have 1 ex-wife).
All in all, the two sentences are not the same to each other.
What do you say for a former wife (or husband) that isn't an ex, but actually deceased?
The following are each a bit different in meaning:
His former wife that is his ex.
His former wife that was his ex and now deceased.
His former, now deceased, wife.