OK, I think you probably need VBA to do that.
Right-click on the tab sheet name at the bottom of the screen, select "View Code", and paste this code in the VB Editor window that pops up:
VBA Code:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rng As Range
Dim cell As Range
Dim r As Long
Set rng = Intersect(Target, Range("F:F"))
If rng Is Nothing Then Exit Sub
For Each cell In rng
r = cell.Row
If Application.WorksheetFunction.CountIf(Range("G:G"), Range("G" & r)) > 1 Then
MsgBox "Entry in column F of row " & r & " causes a duplicate in column G", vbOKOnly, "ENTRY ERROR!"
End If
Next cell
End Sub
Now, if any manual entry you make in column F causes a duplicate to occur in column G, it will give you an error message.
If you want it to also automatically remove the value from column F that you just entered that caused the duplicate value in column G, uncomment these three lines in the code by removing the single-quote mark that is at the front of each line: