Count the Number of Duplicate Characters

Create a function that returns the amount of duplicate characters in a string. It will be case sensitive and spaces are included. If there are no duplicates, return 0.

Examples

duplicates("Hello World!") ➞ 3
# "o" = 2, "l" = 3.
# "o" is duplicated 1 extra time and "l" is duplicated 2 extra times.
# Hence 1 + 2 = 3

duplicates("foobar") ➞ 1

duplicates("helicopter") ➞ 1

duplicates("birthday") ➞ 0
# If there are no duplicates, return 0

Notes

Make sure to only start counting the second time a character appears.