Is it possible to count several ranges of cells, where for one of those ranges I need to count repeating cells only once?

[ziphem]

I wasn't quite sure how to phrase this question, hopefully the title makes some sense. I am completely stumped on this, and would greatly appreciate any guidance!!

I need to count a range of cells (read: columns) based on their values. Ok, no problem there. However, for one of the columns, I am supposed to count unique cell values - meaning, if the cell value repeats itself in several cells (and falls in the range of cells I'm counting), it should only be counted once.

Thus, say I'm counting 100 rows

Column A=Customer ID (e.g., F1023)
Column B=Number of (unique) visits (e.g., 1/2/2012)
Column D=Rep (e.g., John)

Column A Column B Column D Column G
F1023 1/2/2012 John 1/1/2012
F1023 1/3/2012 John 2/1/2012
F3312 1/4/2015 John
F3321 1/5/2012 Mary
F1023 1/6/2012 John
C4450 1/18/2012 Mary
B3320 1/21/2012 John
B3320 1/22/2012 John
B3320 2/5/2012 John
B3320 1/2/2012 John
Veee3 3/3/2012 John
Veee3 1/12/2012 Henry
Veee2 2/1/2012 John
Veee5 1/12/2012 Mark
Ve4 2/12/2012 John

To count the number of visits rep JOHN (Column D) has made to customers (Column B) in January, I can use:

=SUMPRODUCT(($D$1:$D$20="John")*($B$1:$B$20>=$G$1)*($B$1:$B$20<$G$2))

To count the number of customers (Column A) that all sales reps (Column D) have visited , I can use:

=SUMPRODUCT(($A$1:$A$20<>"")/COUNTIF($A$1:$A$20,$A$1:$A$20&""))

But how do I combine both of those? How do I count the number of customers (Column A) that John (Column D) has visited during January (Column B & G)??

Thank you so much!!