Highlight duplicate values

Hello,

I need some help with a macro to highlight duplicate values in different columns that have to the same company ID. so I need 2 conditions in this macro.

1. value in column G match value in column W

and

2. those 2 values belongs to the same comapny = companies are identified in colomun A with a company ID.

I've found this Macro and tried to use but it's highlighting the same values in colomun G and W regardless if they have the same company id or not in colomun A.

Sub HighlightDups()
Dim i, LastRowG, LastRowW
LastRowG = Range("G" & Rows.Count).End(xlUp).Row
LastRowW = Range("W" & Rows.Count).End(xlUp).Row
Columns("G:G").Interior.ColorIndex = xlNone
Columns("W:W").Interior.ColorIndex = xlNone
For i = 1 To LastRowG
If Application.CountIf(Range("W:W"), Cells(i, "G")) > 0 Then
Cells(i, "G").Interior.ColorIndex = 36
End If
Next
For i = 1 To LastRowW
If Application.CountIf(Range("G:G"), Cells(i, "W")) > 0 Then
Cells(i, "W").Interior.ColorIndex = 36
End If
Next
End Sub

I hope someone could help me to edit it as I have no VB or any programing background.

Thank you in advance