Check for duplicate header GO IDs in the 2-D sections variable.

def no_duplicates_sections2d(sections2d, prt=None):
    """Check for duplicate header GO IDs in the 2-D sections variable."""
    no_dups = True
    ctr = cx.Counter()
    for _, hdrgos in sections2d:
        for goid in hdrgos:
            ctr[goid] += 1
    for goid, cnt in ctr.most_common():
        if cnt == 1:
            break
        no_dups = False
        if prt is not None:
            prt.write("**SECTIONS WARNING FOUND: {N:3} {GO}\n".format(N=cnt, GO=goid))
    return no_dups