Modify code to show message box on duplicate entry

Hi everyone.. i am using the following code to enter data into a worksheet. I would like to make it so if the data being entered in textbox1 is already present in columnA of the worksheet then a message box will appear stating so, and none of the data from the form is entered onto the sheet.

Option Explicit
Private Sub CommandButton1_Click()
    Dim LR As Long
    With Sheets("Master")
        LR = .Range("A" & .Rows.Count).End(xlUp).Row
        If LR <> 1 Then LR = LR + 1
        .Range("A" & LR).Value = Me.TextBox1
        .Range("B" & LR).Value = Me.TextBox2
        .Range("C" & LR).Value = Me.TextBox3
        .Range("D" & LR).Value = Me.TextBox4
        .Range("A" & LR).Offset(19, 0).Value = "ENTRY"
    End With
    Unload Me
End Sub