excel - divide duplicate values by the number of duplicates for that value -

i want write vba code or excel formula divide duplicate values number of duplicates value. example a- 6 b- 8 c- 3 d- 4 f- 5 a- 8 a- 4

the code add 6+8+4 , divide 3. replace each current value 6.

meh...

say column contains letters, column b contains numbers, , row 1 header row. enter formula in cell c2 , pull down:

=b2/countif(a:a,a2)