unique-name-generator generates incorrect names, which can cause broken queries

[camsaul]

(let [unique-name (mbql.u/unique-name-generator)]
  [(unique-name :x "A")
   (unique-name :x "B")
   (unique-name :x "A")
   (unique-name :y "A")
   (unique-name :y "A")])
;; => ["A" "B" "A" "A_2" "A_3"]

but it should generate

(let [unique-name (mbql.u/unique-name-generator)]
  [(unique-name :x "A")
   (unique-name :x "B")
   (unique-name :x "A")
   (unique-name :y "A")
   (unique-name :y "A")])
;; => ["A" "B" "A" "A_2" "A_2"]