Make sure config true leaf-lists do nothave duplicate defaults

def v_unique_name_leaf_list(ctx, stmt):
    """Make sure config true leaf-lists do nothave duplicate defaults"""

    if not stmt.i_config:
        return
    seen = []
    for defval in stmt.i_default:
        if defval in seen:
            err_add(ctx.errors, stmt.pos, 'DUPLICATE_DEFAULT', (defval))
        else:
            seen.append(defval)