In casual conversation, "I don't fit in in this world" would be understood, and many native speakers might say the exact same sentence. However, if this were a written sentence, it would be a good idea to revise the sentence to remove the awkward repetition. Here are some possible revisions:
I don't fit in with this world.
I don't fit into this world.
In this world, I don't fit in.
This kind of repetition can occur with other phrasal verbs. Here are some examples:
The conflict will blow over over the weekend.
Here are some clothes you can try on on the runway.
Could you pitch in in the kitchen?
Again, these are all "ok" grammatically, but they are poor choices stylistically.
Your pronoun example does not quite make sense. It seems to be a run-on sentence. I'm not sure if these alternatives have exactly the meaning you intend, but you could try:
There's no limit. If you think about it, it exists.
There's no limit if you think about it. It exists.
However, some sentences could have repeated pronouns in this way:
The robot's wheels were covered in dirt. Behind it it left dark tracks on the carpet.
Again, this might be a correct sentence, but it would be much better to rephrase the sentence to remove the awkward "it it", even with a simple comma between the two.