"""
Given a string, you have to return a string in
which each character (case-sensitive) is repeated once.

double_char("String") ==> "SSttrriinngg"
double_char("Hello World") ==> "HHeelllloo  WWoorrlldd"
double_char("1234!_ ") ==> "11223344!!__  "

Good Luck!
"""

def double_char(s):
    output = ''
    for i in range(len(s)):
        output += s[i]*2
    return output

print('Tests:')
print(double_char("String"))
print(double_char("Hello World"))
print(double_char("1234!_ "))