« Personne ne l’a pas fait » veut dire « tout le monde l’a fait ».

En maths, la négation de “pour tout x, on a x > y” par exemple, c’est “il existe un x tel que x < y”.
Le raisonnement se tient à l’identique ici, du coup :

• “Tout le monde l’a fait” deviendra “Il y a au moins une personne qui ne l’a pas fait”
• “Il n’y a pas de gens qui l’a fait” (dit autrement, “Personne ne l’a fait”) deviendra “Au moins une personne l’a fait”.

Ne can combine with many other negation words as explained in this anwser, and it will form a single negation. Here is an extreme example with 5 surrounding negation words:

Personne ne dit plus jamais rien à personne.

That is: “nobody ever says anything to anyone else anymore.”

However pas cannot normally be part of a larger combination of surrounding words. If you turn the plus into a pas:

Personne ne dit pas jamais rien à personne.

The sentence becomes ungrammatical and utterly incomprehensible in this case. People may try to interpret it as a double negation, but it is not clear how. It could however sometimes be used in conversation to disagree with someone who previously said “Personne ne dit jamais rien à personne”. In this case it would be interpreted as “You’re wrong. It is not the case that nobody ever says anything to anyone else.

Your example,

Personne ne l’a pas fait.

sounds a bit weird because it’s similarly ungrammatical. Though people can understand it as a different kind of cancelling double-negation: “nobody did not do it”, which sounds similarly weird but is easier to parse.