Does this mean anything more substantial in Arabic than the English translation suggests?
Actually, i would put that in Arabic as: المدينة هي المدينة (al-madínatu hiya-l-madínah) for emphatic influence.
When a word is used repetitively like that, in Arabic, it is an expression noting that this is a thing this is how it is. In other words, if I want to rephrase the sentence above in English it would be like: this is the city, it didn't change.
Another example: الرجل هو الرجل (the man is the man) - i.e. the man doesn't or didn't change (can be general concept of the man or simply talking about a specific man). The emphatic pronoun between the two words can be removed but I think the expression that way is weak, but understandable I would say.
Why don't we say "tu" at the end of اَلْمَدينة? I thought it was orally said when it is a definite nominative case but I maybe missed sth :)
well the record here is not the best - however, we can assume that there is a pause between the two words (the sentence doesn't make much sense but it's Duolingo anyway).
At the end of a phrase or a sentence or when the word is isolate, the Ta-Marbuta at the end (and vowels in general) can be dropped because there is no need to add them (adding them is mainly for connecting this part of the speech with whatever is after them). So, a "tu" can simply remain "H"
Here is post with an audio, which might be useful for you to read, if you like: https://forum.duolingo.com/comment/33566303