man manna min mannin munna min l-mannaan
Some natives might have trouble understanding it even with the diacritics 
This ّ is called shadda, it's written above a consonant to indicate that it's doubled. What that actually means is that the consonant should be double the length in pronunciation, e.g.:
فَهَّم
fahham to explain (make someone understand), pronunciation
spoiler
The shadda happens when a consonant occurs twice without a vowel in between. And it changes the meaning of the word of course.
Here is a calligraphic writing of the tongue twister in riq3a
Who wants to learn about the remaining two short vowels?
Is manna etymology related to mana like the word mana from heaven?
Yeah turns out that's a popular theory actually, I had no idea.
Interesting read!