If you are in one part of Israel, and celebrate Purim on the normal 14th of Adar, but then end up being in Jerusalem in time to celebrate Shushan Purim, are you obligated to celebrate Shushan Purim even though you've already fulfilled normal Purim? If you're not able to celebrate for whatever reason (business, travel, something like that), would that be an issue?
When I went to Eretz Yisrael (in 5727), I was told (by Rabbi Besdin Z'tzl of YU) that it depends on where you normally live. If you are living in Yerushalayim and happen to be elsewhere, then you should (if you can) return to Yerushalayim for Shushan Purim. However, you can attend the megilla reading where you happen to be especially if you are not sure that you can make it back. Once you are Yotzi, then you do not have to fulfill Shushan Purim. On the other hand, I know of people who go to an outside community for Purim and then back to Yerushalayim for Shushan Purim (even though they do not have to) because they want to be part of the celebrations in both places.
As always, check with your rav and consider what your family minhag might be.
I just found the exact halachos of someone who is in one place or the other in פסקי תשובה by Rabinowitz starting at דף תקכט which goes through all the details and various circumstances.