Using an If/Then formula with an Index and Match

[ChantellMaree]

Alright, so I'm pretty new to excel and am trying to create a formula to save me some time.
Basically I have to make sure our orders are on time. I get a spreadsheet each week with information from where we ordered our parts from, and I have to compare the dates they gave me last week, with the ones I have this week to see if there is a variance.

The problem that I'm having is that I'm doing Index and Match with an Array.
So I have it look for both the order number and the part that was ordered to return back to me the date in which is listed.
BUT. There are some instances that there are duplicate order number and part numbers, so it just returns back to me the first date it finds instead of all of them.

I was told that I need to have an IF/Then function to basically say, IF there are duplicate order numbers/part number combo then return all the dates.
I believe I also need to use a count, but I have no idea how to do this or where to put it in the formula.
The current formula I have is: =INDEX('last week'!$G$2:$G$169,MATCH(current!$A$2&current!$B$2,'last week'!$A$2:$A$169&'last week'!$B$2:$B$169,0))

Any help would be appreciated, and if I need to clarify I can.

Thank you!

 

[AhoyNC]

Maybe something like this:
This is an array formula and must be entered with CTRL-SHIFT-ENTER.
Drag formula down as needed.

Excel Workbook
	A	B	C	D	E	F
1	Order #	Part #	Date		Order #	Or4
2	Or1	Par1	5/1/2018		Part #	Par5
3	Or2	Par2	5/2/2018		Date(s)	5/5/2018
4	Or1	Par1	5/3/2018			5/7/2018
5	Or4	Par4	5/4/2018
6	Or4	Par5	5/5/2018
7	Or6	Par6	5/6/2018
8	Or4	Par5	5/7/2018
9	Or8	Par8	5/8/2018
10	Or9	Par9	5/9/2018
Sheet