python_ extracted strings to appear twice

# Extracted strings to occur twice, using statistical count 
DEF two_zifuchuan (STR): 
    S = SET ()
     for I in STR:
         IF str.count (I) == 2 : 
            s.add (I) 
    return S 

# extracted strings to occur twice, using statistical dictionary
 DEF two_occur (STR): 
    S = {}
     for I in STR:
         IF I in s.keys (): 
            S [I] +. 1 =
         the else : 
            S [I] =. 1
     return [I for I in s if s[i]==2]

str="dddredddddewws22dff43"
print(two_zifuchuan(str))
print(two_occur(str))