Using an Array Across two SubFunctions in VBA

I'm writing a macro that compares two columns of data and then identifies the rows where there is duplicate data found across both columns. That part of my program works. However, I dont know how to use arrays across two separate "Subs" in VBA. It's easier to explain if you first see my code.

Function DuplicateFinder(SheetName1 As String, SheetName2 As String)

Dim D As Object, C
Dim nda As Long, ndb As Long
Dim test As Range
Dim StorageArray(1000)
Dim increment
increment=0   

Set D = CreateObject("scripting.dictionary")
Sheets(SheetName2).Select
ndb = Range("O" & Rows.count).End(xlUp).Row
Sheets(SheetName1).Select
nda = Range("O" & Rows.count).End(xlUp).Row

For Each C In Range("O2:O" & nda)
    D(C.Value) = 1
    C.Select
Next C

Sheets(SheetName2).Select
For Each C In Range("O2:O" & ndb)
    If D(C.Value) = 1 Then
        C.Select

        StorageArray(increment) = C.Value ' this is where i want to store the C value.
    End If
    If Len(C) = 0 Then
        C.Interior.Color = vbRed
        MsgBox "Macro terminated at the blank red cell," & Chr(10) & _
            "as per instructions"

    End If
Next C

End Function

Sub MainFunction()

Dim A As String
Dim B As String
Dim C As String
Dim D As String

A = "Sheet 1 Name"
B = "Sheet 2 Name"
C = "Sheet 3 Name"
D = "Sheet 4 Name"
increment = 0


Call DuplicateFinder(Sheet 1 Name, Sheet 2 Name)
'I would then call the function 5 more times to compare each column in each sheet to one another

End Sub

The first function is used to compare the data across column '1' and column '2', and then identify the cells where there is duplicate data across each column. Again, that part works. The second sub is just the main function used to run the code. What I want to do, and don't know how to, is every time the DuplicateFinder finds a duplicate, it saves that 'data' in an array. However, I need to run the DuplicateFinder Function 6 times to compare the data across each sheet in my workbook. For example, if the sheets name's were A, B, C, and D. I need to run the function that compares A to B, A to C, A to D, B to C, B to D, and finally C to D. However, the data saved in the array is only available in the DuplicateFinder Function.

I was thinking maybe the solution was to have the function return the value, but I don't understand how that works. I would appreciate anyone's input.