def duplicate_header(header, duplicate):
    """Create and return an 'Duplicate header' error string"""
    ctx = duplicate.start_context()
    octx = header.start_context()
    errmsg = 'Duplicate header, {}{}'.format(header.name, ctx)
    if len(octx) > 0:
        errmsg = errmsg + ', original{}'.format(octx)
    # End if
    return errmsg