In English it is very common to use the apostrophe followed by an "s" to show possession. This is especially true with names. However, many speakers do not add an "s" with names which end in an "s."
For example, many speakers say "James' room" instead of "James's room." Though the form "James's" is possible, many people do not use it. With names which end with one consonant in the coda, however, the apostrophe followed by "s" is common. For example, many people say "Liz's room."
In my case, I use an "s" after the apostrophe if a name has one consonant in the syllable coda This is the case with the name "Liz." With a name which has two consonants in the coda, I add an "s" if the final consonant is voiceless. This is the case with the name "Lance." I add an "s" and say "Lance's room." However, with a name such as "James," the final consonant is voiced. For this reason, I do not add an "s."
The use of an apostrophe and an "s" with names that end with "s" varies from speaker to speaker. In my case, I use an "s" with all monosyllabic names. For names of more than one syllable, I use an "s" for names that have one consonant in the coda and two consonants in the coda in which the final one is voiceless. For me, the lone exception to this rule appears to be the name "Jesus."