Counting Duplicates

[hic003cih]

package kata

import "strings"

func duplicate_count(s1 string) int {
    counter := make(map[string]int)

    for _, s := range s1 {
      sl := strings.ToLower(string(s))
      counter[sl] += 1
    }

    sum := 0
    for _, v := range counter {
      if v > 1 {
        sum += 1
      }
    }

    return sum
}