Try a little experiment: stand in the middle of any Cairene street and loudly call for Mohamed, in a tone that indicates familiarity and slight irritation. Chances are, a large portion of the male population will look to respond to you.
Now a harder experiment: obtain the student roster of Cairo University and in a randomly chosen class, count the number of names made up of combinations of Ahmed and Mohamed: a large percentage again. Look at the rest: mostly made up of combinations of first names.