Sub Test()
Dim Feuille As Worksheet
For Each Feuille In ThisWorkbook.Worksheets
If Feuille.Name = Format(Date, "yyyy-mm-dd") Then
MsgBox "La feuille du jour existe déjà !", vbCritical + vbOKOnly, "ATTENTION !"
Exit Sub
End If
Next Feuille
Sheets(1).Copy Before:=Sheets(1)
Sheets(1).Name = Format(Date, "yyyy-mm-dd")
End Sub