Sub Essai()
Dim e, derlig As Long, c As Range
With CreateObject("Scripting.Dictionary")
.CompareMode = 1
For Each e In [{"Janvier","Février","Mars","Avril","Mai"}]
derlig = Sheets(e).Range("B" & Rows.Count).End(xlUp).Row
If derlig > 1 Then
For Each c In Sheets(e).Range("B2", Sheets(e).Range("B" & Rows.Count).End(xlUp))
'.Item(c.Value) = IIf(.Exists(c.Value), .Item(c.Value) + 1, 1)
If c.Value <> "" Then .Item(c.Value) = Empty
Next c
End If
Next
If .Count > 0 Then
Sheets("Synthèse").Range("A2").Resize(.Count).Value = _
Application.Transpose(.keys)
End If
End With
End Sub