Editors disagree on many things, including how to form the possessive of a name ending in S. The folks behind the Associated Press style book advocate adding just an apostrophe. So, the possessive of Spears is Spears’ if you follow AP style. The editors at the Chicago Manual of Style recommend adding an apostrophe and an S: Spears’s.
But nowhere is there more violent disagreement than among the editors for the Yahoo! front page, who can’t agree among themselves on the possessive of Spears. The rest of the editing world agrees that using both forms on the same page, in the same module, makes them look like total incompetents: