Consider these example sentences given by my grammar dictionary:
Observation: The location is marked by へ instead of に.
Considering that a sentence such as
デパートに行く is grammatical,
if the locations in (1) and (2) are marked by に, two にs would be present in the same sentence.
(Question): Can the location in (1) and (2) be marked by に? Would two にs cause the sentence to be ungrammatical?