Option Explicit
Sub test()
Dim a, b(), dico As Object, ws As Worksheet, n As Long, i As Long
Set dico = CreateObject("Scripting.Dictionary")
dico.CompareMode = 1
'attention à la 1ère dimension
ReDim b(1 To 100, 1 To 2)
For Each ws In Worksheets([{"Feuil1","Feuil2","Feuil3"}])
a = ws.Cells(1).CurrentRegion.Value
For i = 2 To UBound(a, 1)
If Not dico.exists(a(i, 1)) Then
n = n + 1
b(n, 1) = a(i, 1)
dico(a(i, 1)) = n
End If
b(dico(a(i, 1)), 2) = b(dico(a(i, 1)), 2) + a(i, 2)
Next
Next
With Sheets("Feuil5").Cells(1)
.CurrentRegion.Clear
.Resize(1, 2) = Array("Utilisateurs", "Volume")
.Offset(1).Resize(n, UBound(b, 2)).Value = b
End With
Set dico = Nothing
End Sub