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?
