question, I don't want the second msgbox activated.

[Belleke]

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Intersect(Target, Range("A1")) Is Nothing Then
    MsgBox "Maak eerst je keuze in cel A1", vbExclamation, "Cel A1 is leeg"
End If
End Sub

 

[vletm]

Belleke​

Have You tested it with Application.Enabled?
... before If set it to False
and
... after if set it to True

 

[Debaser]

What second message box? Do you mean if you select A1 more than once?

If you mean you only want to see it once per session, you could use a static variable. If you mean you only want to see it once ever, you'd have to store a value somewhere that you can check, eg in a named range, registry, text file, etc.

 

[Belleke]

Example

 

[vletm]

Belleke​

... seems You haven't.
This my sample works other way.
Did You miss to explain - how something should work?