If something has been authorized, then necessarily it was properly authorized. If the appropriate procedures hadn’t been followed, then no authorization would have been granted. Similarly, if, say, an imposter signs on behalf of a party, the contract won’t have been executed signed by that party.
That’s why Garner’s Dictionary of Legal Usage, at 301, says, with respect to duly authorized, “Because authorize denotes the giving of actual or official power, duly (i.e., ‘properly’) is usually unnecessary. Likewise, duly is almost always redundant in phrases such as duly signed.”