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

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.