An important note: In my limited experience, many native Korean speakers may not know that it is often necessary to distinguish the USA government capital, Washington D.C. from Washington.
For example, they may not realize Seattle, Washington is not located anywhere near Washington D.C.
For most people, it is not one of those things worth knowing about until it is necessary. It simply would be more likely to come up when speaking to Americans. So, an American should consider the inverse when speaking to Koreans (or to include your comment, folks from other countries).
non american here. i had no idea where washington dc actually was until i finally googled it, and then it made sense. i was so confused thinking why is the capital all the way up north near seattle, makes no sense. even though i really should ve figured it out from the d.c part. silly.
there is no need to double ㅅ to get sa, seo, so, su, sae and se sounds. if you pay some attention, the pattern to get [sh] is in most cases ㅅ + y (vowel). without y, ㅅ is pronunced as [s].
사 = ㅅ + ㅏ = sa
샤 = ㅅ + ㅑ = sha
as for 시, even if you double ㅆ, it is still pronunced [shi].