Simple way to build a dictionary that has a list of lists as values. Good for removing redundancies (keys) but preserving redundancies in the values.

simple dictionary creation

d = {}
with open('myfile.ext','rU') as f:
  for line in f:
    linesplit = line.split('\t')
    try:
      d[linesplit[fasta_pos]].append( [x for x in linesplit[other_positions] ) #list comprehension 
    except KeyError:
      d[linesplit[fasta_pos]] = []
      d[linesplit[fasta_pos]].append( [x for x in linesplit[other_positions] )

# write to file:
out = open('fileout.ext', 'wb')
for key in d.keys():
  # format key and d[key] appropriately
  out.write(output)