Count and use uniques

Counter implementation

from collections import Counter

# construct counter to count the number of occurances of each fasta
cnt = Counter()
with inputfile as f:
  for line in f:
    linesplit = line.split('\t')
    cnt[linesplit[fasta_pos]] += 1 #establish counter, count occurrances of each fasta file

with inputfile as f:
  for line in f:
  linesplit = line.split('\t')
  if cnt[linesplit[fasta_pos]] == 1: #look at your counter, look at how many times the fasta occurs
    outfile.write(......)