Questions & Answers about W niedzielę w mieście jest mniej ludzi.
Because the two w phrases do two different jobs:
- w niedzielę = on Sunday
- w mieście = in the city
So it is not repetitive in a bad way; Polish simply uses the same preposition w for both a time expression and a place expression here.
Niedziela is the dictionary form, or nominative.
After w in this kind of time expression with days of the week, Polish uses the accusative:
- w niedzielę = on Sunday
- w środę = on Wednesday
- w sobotę = on Saturday
So w niedzielę is just the correct case form after the preposition in this expression.
Because after w when it means in a place, Polish uses the locative case.
- miasto = city, town
- w mieście = in the city / in town
So the noun changes form:
- miasto → mieście
This is a normal case change after w for location.