Níl a fhios ag duine ar bith acu

Why is the preposition ag used twice in this sentence: ag + acu?

Níl a fhios ag duine ar bith = Nobody knows
Níl a fhios ag duine ar bith acu = None of them knows

Edit: I had this in the wrong tense but I've corrected it now.

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Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

The acu in duine bith acu is the acu in cé acu that we discussed in your other thread here the other day.

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).