Prevent duplicate entry in userform

try

Private Sub cmdAdd_Click()
    Dim RowCount As Long
    Dim xlRng As Range
    
    If Trim(Me.Ac.Value) = vbNullString Then
        MsgBox "Please enter Ac.", vbOKOnly + vbInformation, "Error"
        Me.Ac.SetFocus
        Exit Sub
    End If
    
    Set xlRng = Worksheets("Sheet1").Columns(1).Find(What:=Me.Ac.Value, LookIn:=xlValues, lookat:=xlWhole)
    If Not xlRng Is Nothing Then
        MsgBox "Duplicate Ac.", vbOKOnly + vbInformation, "Error"
        Me.Ac.SetFocus
        Me.Ac.SelStart = 0
        Me.Ac.SelLength = Len(Me.Ac.Value)
        Exit Sub
    End If
    
    RowCount = Worksheets("Sheet1").Range("A1").CurrentRegion.Rows.Count
    
    With Worksheets("Sheet1").Range("A1")
        .Offset(RowCount, 0).Value = Me.Ac.Value
        .Offset(RowCount, 1).Value = Me.Cl.Value
        .Offset(RowCount, 2).Value = Me.Ti.Value
        .Offset(RowCount, 3).Value = Me.S1.Value
        .Offset(RowCount, 4).Value = Me.S2.Value
        .Offset(RowCount, 5).Value = Me.Bo.Value
        .Offset(RowCount, 6).Value = Me.Ro.Value
        .Offset(RowCount, 7).Value = Me.La.Value
        .Offset(RowCount, 8).Value = Me.Pu.Value
        .Offset(RowCount, 9).Value = Me.Ye.Value
        .Offset(RowCount, 10).Value = Me.Ed.Value
        .Offset(RowCount, 11).Value = Me.Isbn.Value
        .Offset(RowCount, 12).Value = Me.PR.Value
        .Offset(RowCount, 13).Value = Me.pg.Value
        .Offset(RowCount, 15).Value = Me.Lo.Value
    
    End With
    RefreshSOUL
End Sub