Sum if unique and another criteria

Thankyou for everyone's help yesterday it was great. I have one last
thing that is bafaling me to do with the same thing.

I have a column in D that shows the number of days an invoice is over
due


| A | B | C
| D |
10 N/A yes
0
10 N/A yes
0
11 N/A
no 2
11 N/A
no 2
12 N/A yes
0
13 N/A
no 2
13 N/A
no 2
13 N/A
no 2

I wish to sum the number in coulmn D for those that are overdue (no in
Column C) and that are unique entries (column A, invoice number), in
this example there are 4 unique entries.

With the help from everyone yestarday i have manged to be able to sum
up the number of invoices that were overdue (Column C=no), so in the
obove example the result would be 2 (invoice 11 and 13) by useing this
array formula.

=SUM(N(FREQUENCY(IF(C:C="No",MATCH(A:A&"",A:A&"",0 )),MATCH
(A:A&"",A:A&"",0))0))

I am at a loss at how i can then extend this to sum up the same
corrospnding values in column D.

Oh and i then need to also devide this by the total number of invoices
that failed.

Hope someone can help.

Many Thanks
Gareth