conjugation eter verbs

A quiz question asked, 'What are the two conjugations of "you buy" (using vous singular and vous plural). Answer choices include: 1)Tu achetes (with an accent grave over the first e), vous achetez and 2)tu achettes, vous achetez. The correct answer is 1 but why is the t not doubled? According to the notes ex. projeter was doubled for tu but not for vous. Also, why was there an accent over the 't'?