If you have duplicates and don't want to count the duplicates.
For example if the highest number is 4 but there are two fours in this column, and down't want to count the second 4 as the second highest number, perhaps:
For the 2nd highest number:
=LARGE(G1:G5,1+COUNTIF(G1:G5,MAX(G1:G5)))
For the 2nd lowest number:
=SMALL(G1:G5,1+COUNTIF(G1:G5,MIN(G1:G5)))