Ensure a set of input VCFs do not have duplicate samples.

def _check_samples_nodups(fnames):
    """Ensure a set of input VCFs do not have duplicate samples.
    """
    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))