The AP Stylebook, The Chicago Manual of Style, the Merriam-Webster Guide to Punctuation & Style, and The New York Times Manual of Style and Usage suggest omitting the S after the apostrophe for certain singular possessives when the singular possessive already ends in S and comes before a word that begins with S.
The Chicago Manual of Style and the New York Times Manual of Style and Usage suggest omitting the S after the apostrophe for certain singular possessive proper nouns that end in S, particularly historically significant figures.
The New York Times Manual of Style and Usage suggests omitting the S after the apostrophe for certain singular possessives that end with "two sibilant sounds separated only by a vowel sound." According to The New York Times Manual of Style, sibilant sounds include sounds represented by the letters CH, J, S, SH, TS, and Z.