There are three sentences. Here is the first one:
- Laura ist in dem Haus. (Laura ist im Haus.)
Das Haus has a neuter grammatical gender, dative case is used, so it is im. That is the rule.
Here is the second sentence:
- Johannes ist in Berlin.
Berlin has also neuter grammatical gender. The way I understand it, proper nouns do not change the article in into im in the dative case (if they have neuter or masculine grammatical gender). That would probably be an exception to the already mentioned rule.
Here is the third sentence:
- Es gibt keinen Eintrag „pragerisch“ im Duden.
Duden is a proper noun. According to dict.cc it has a masculine grammatical gender. (Duden doesn't have an entry for the word Duden, but it has probably really the masculine grammatical gender). The word Duden is a proper noun, and because of it there should be in Duden.
I am thinking of three possible explanations:
i) The word Duden is an exception of the exception.
ii) Proper nouns having masculine grammatical gender do have the im preposition in dative case.
iii) The sentence 3. is wrong and there should be in Duden.
Could you please help me to figure out the situation?