Generating k+1 length strings from a list of k length strings:Need a better method

[girish_sahani]

I have a list of strings all of length k. For every pair of k length
strings which have k-1 characters in common, i want to generate a k+1
length string(the k-1 common characters + 2 not common characters).
e.g i want to join 'abcd' with bcde' to get 'abcde' but i dont want to
join 'abcd' with 'cdef'
Currently i'm joining every 2 strings, then removing duplicate characters
from every joined string and finally removing all those strings whose
length != k+1.Here's the code i've written:

def withoutDup(string):  #check
    seen = {}
    chars = []
    for c in string:
        if c not in seen.keys():
            chars.append(c)
            seen[c] = 1
            continue
        return ''.join(chars)

    def genColocations(prunedK):
      prunedNew = prunedNew1 =  subsetList = []
      for i in range(0,len(prunedK) - 1,1):
        for k in range(1,len(prunedK),1):
            if i+k <= len(prunedK) -1:
                colocn = prunedK[i] + prunedK[i+k]
                prunedNew1.append(colocn)
                continue
            continue
        for string in prunedNew1:
            stringNew = withoutDup(string)
            prunedNew.append(stringNew)
            continue

But it is quite bad in the time aspect :(. Please help me out.

Thanks in advance,
girish