Option Explicit
Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
If Target.Column = 1 And Target.Value <> "" Then
MsgBox "Une séance existe déjà à cette date"
Exit Sub
Else
If Target.Column = 1 Then Target.Value = Date: Cancel = True
If Not Intersect(Range("A3:A" & Range("A" & Rows.Count).End(xlUp).Row), Target) Is Nothing Then
If Not IsError(Application.Match(CSng(Date), Columns("A"), 0)) Then
MsgBox "Une séance existe déjà à cette date"
Target = ""
End If
Cancel = True
ElseIf Not Intersect(Range("A3:A" & Range("A" & Rows.Count).End(xlUp).Row), Target) Is Nothing Then
End If
End If
End Sub