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

Here is a calligraphic writing of the tongue twister in riq3a

Who wants to learn about the remaining two short vowels?