You know the LARGE function which can find the nth largest value. Hence, if you have a series like below -

1

And you give =LARGE(A1:A10,3), you get the answer as 18

Now, if we have a series like below

1

Now, you give =LARGE(A1:A10,3) and now the result is 24. The reason is that large function gives the nth largest value in a sorted array. Hence, LARGE function will sort the above array as {24,24,24,22,22,18,18,9} and 3rd largest is 24.

But actually you want the unique 3rd largest which is 18 as the answer.

The formula for such case would be

=LARGE(IF(FREQUENCY($A$2:$A$10,$A$2:$A$10)<>0,$A$2:$A$10),3)