How to generate k+1 length strings from a list of k length strings?

[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:

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

But this one is quite bad in the time aspect :(.
Thanks in advance,
girish