Patenting an invention in the U.S. that may be patented elsewhere

First you need to understand what a patent does NOT do. A patent DOES NOT give you the right to make, use, or sell your invention. A US patent only gives you the right to exclude others--except the US Government--in the US and it's territories from making, using, and selling or importing your invention. An owner of an enforceable prior patent that is broader than yours and which reads on your invention can stop you, various laws can stop you, and the government can stop you from making, using, and selling your own invention. Your US patent has no effect outside the US or its territories and vice versa. Patents are territorial in nature. Only if the other person whose patent you want to patent in US has not filed any PCT with US being one of the designated countries, then you can very well patent the same in US otherwise not.

Not usually. There may be some differences between your invention and what has already been patented, but if they are substantially the same, you cannot normally get a patent here in the US - there are some exceptions, dates come into play, inventors come into play, but normally if A has a patent in Japan, and B wants to patent the same invention here in the US, and A's patent is already more than one year old, you cannot obtain your patent here in the US.

Patent claims may be rejected if the same (or similar enough) invention was previously patented in this country or in any foreign country. So unless you have something in common with the “someone else” (e.g. common ownership or common inventorship) the patent claims may be rejected in the United States. Furthermore, even if the USPTO misses the prior art during your examination and you were able to get a patent, your patent could be later invalidated if the prior art in the other country is discovered by a third party. Therefore, the best approach may be to improve upon what has already been invented elsewhere and claim the improvement.

If minor variation, consider the petty patent (sometimes called innovation patent) instead. However you may still need freedom to operate clearance. But the world is moving towards absolute novelty standard so trying to arbitrage between PCT zones leaves you vulnerable to inter partes review which could disallow ALL claims.