Count the number of times a value appears in a column using Ms Access

Hi experts,

How can we accomplish the following in Access.

Situation:
We have a column with an internal codes to identify trees. This internal code is unique for each tree.
But in cases where that tree is divided in 2 or 3 pieces, that internal code gets repeated in that column.

Now for reporting it is needed that we know how many trees there are and how many pieces.

How can we count each unique internal tree code?
I would like to see a column that shows for each record one of the following values: 1 or 0.5 or 0.33.

The values are determined by the counting of the internal tree code.
So if the counting result is
1, show value"1"
2, show value "0.5"
3, show value "0.33"


Am looking forward to a solution.