An export that ran twice doubles every row, and a duplicated invoice overstates revenue without anything looking wrong. `Range.RemoveDuplicates` is one line; the useful version around it does two more things. It keys on the columns that actually define a duplicate — the invoice number, not the whole row, which can differ by a timestamp — and it counts the rows before and after, so you know how much it removed. A dedupe that doesn't report its count is just unexplained data loss.

Sub DedupeInvoices()
    Dim ws As Worksheet, rng As Range
    Dim before As Long, after As Long, lastRow As Long, lastCol As Long

    Set ws = ActiveSheet
    lastRow = ws.Cells(ws.Rows.Count, 1).End(xlUp).Row
    lastCol = ws.Cells(1, ws.Columns.Count).End(xlToLeft).Column
    If lastRow < 3 Then Exit Sub

    Set rng = ws.Range(ws.Cells(1, 1), ws.Cells(lastRow, lastCol))
    before = lastRow - 1

    ' Key on column 1 (invoice number) only. Array(1, 3) would key on two.
    ' Header:=xlYes stops the header row being treated as data.
    rng.RemoveDuplicates Columns:=Array(1), Header:=xlYes

    after = ws.Cells(ws.Rows.Count, 1).End(xlUp).Row - 1

    MsgBox "Removed " & (before - after) & " duplicate row(s)." & vbNewLine & _
           before & " rows before, " & after & " after.", vbInformation
End Sub

Paste into the Visual Basic Editor (Alt + F11 → Insert → Module), then run and save as macro-enabled (.xlsm).

• Clean an export that ran twice.
• Reduce a transaction list to one row per invoice.
• Build a unique customer list from a year of orders.

A simulation — press Run to perform what the macro does to a sample workbook.

• Data → Remove Duplicates — The same thing by hand, and it reports the count too. The macro is for doing it every month without thinking. See the tip.
• UNIQUE — Returns a de-duplicated copy without touching the source — safer when you want to keep the original.