Easy once you think about it

               ♠ 7 4 3
               ♥ K J 4
               ♦ A 9 7 6 3
               ♣ 10 4
♠ 9 6 2                      ♠ K J 10 8 5
♥ 10 9 7 2                   ♥ A 6
♦ 4 2                        ♦ Q J 10 8
♣ Q 8 7 2                    ♣ 6 3
               ♠ A Q
               ♥ Q 8 5 3
               ♦ K 5
               ♣ A K J 9 5

       West   North   East   South
                      1♠     Dbl
       Pass   2♦     Pass    2NT
       Pass   3NT     All Pass

When South doubled and rebid notrump, it showed a hand that was too strong for a 1NT overcall originally, and North had an easy 3NT bid.

West led the ♠2 to the king and ace. South led a heart to the ♥J and ace. East led another spade and South won his queen.

Declarer then tested hearts, and, when they didn't split, he took the club finesse. West won the ♣Q, led his last spade, and South was down one -- very unlucky.

Do you see where declarer went wrong?

The 100% line (given the opening bid) is to lead to the ♦A at trick two, and advance the ♥4. If East wins his ace, South has nine tricks via two spades, three hearts and two tricks in each minor suit.

So, suppose East (correctly) ducks. Now South switches gears. He leads the ♣5, not caring who wins it. This gives him: two spades, one heart, two diamonds, and four clubs for a total of nine tricks.

Easy once you think about it.