I like Your solution. I made it somehow similiar to what you did. I add that I learn python 3rd day, so if someone see the problem in the code, all the comments are much appreciated.

def merge_the_tools(string, k):

for i in range (int(len(string)/k)):
    s = string[i*k:(i+1)*k]
    result = ''
    for j in range (0 ,k):
        if s[j] not in result:
            result += s[j]
    print (result)