according to this you can do both

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However, if the word that takes the apostrophe is a name which ends in the letter ?s?, there are two possibilities. Take for example the name Peter Jones, which ends in an ?s?. If we are talking about his house, we can say Peter Jones?s house, spelt J-O-N-E-S-apostrophe-S, or we can say Peter Jones? house, spelt J-O-N-E-S-apostrophe. This second use, the one without the letter ?s?, is more common in formal writing.

Posted By: SCC 28, Nov 18, 15:25:13