The world in which we live in is full of ice cream.
The world in which we live is full of ice cream.
The world we live in is full of ice cream.
I think what’s happening in the problem sentence above is a kind of garden path effect. Specifically, the writer is in the process of writing the sentence, writes “in which”, and then by the time s/he gets to the end of “we live”, working memory fails, and another “in” is inserted. Oops!
Since English allows us to use prepositions either as post-verbal particles (or adverbs) or as true prepositions, we have two options when we create a relative clause. The first solution sentence gives us one option: a preposed preposition with a relative pronoun; and the second solution sentence gives us the other option: no relative pronoun of any kind, and the preposition “in” used adverbially. If a writer forgets (working memory, remember) which strategy they’ve chosen, though, problems like this can occur. It happens to everyone every now and again, so try to anticipate troublesome errors like this one.