code: def _check_samples_nodups(fnames):
      
      counts = defaultdict(int)
      for f in fnames:
          for s in get_samples(f):
              counts[s] += 1
      duplicates = [s for s, c in counts.items() if c > 1]
      if duplicates:
          raise ValueError("Duplicate samples found in inputs %s: %s" % (duplicates, fnames))
docs: Ensure a set of input VCFs do not have duplicate samples.