vba loop no checking for duplicate part number - excel

Question

I need my loop to check for existing part numbers and only if there is no existing part number to add it to my table. If the part number already exists, to have a message box stating that it already exists. Its adding it to my table just fine, but will not give me the message box if there is already an existing part number.
Private Sub Add_Click()
Dim ws As Worksheet
Set ws = Sheet4
Dim X As Integer
Dim lastrow As Long
Dim PartColumnIndex As Integer
Dim DescriptionColumnIndex As Integer
Const Part = "CM ECP"
Const Description = "Material Description"
Dim PartNum As String
Dim MaterailDescription As String
Dim tbl As ListObject
Set tbl = ws.ListObjects("Master")
Dim newrow As ListRow
Set newrow = tbl.ListRows.Add
With ws
On Error Resume Next
Let PartColumnIndex = WorksheetFunction.Match(PartNum, .Rows(2), 0)
Let DescriptionColumnIndex = WorksheetFunction.Match(MaterialDecription, .Rows(2), 0)
Let lastrow = .Cells(.Rows.Count, PartColumnIndex).End(xlUp).Row
X = 3
Do
Let PartValue = .Cells(X, PartColumnIndex).Value
Let DecriptionColumnIndex = .Cells(X, DecriptionColumnIndex).Value
If TextBox1.Value = PartValue Then
MsgBox "Part Number " + TextBox1.Value + " already exists. Please try again or return to main screen."
ElseIf TextBox1.Value <> PartValue Then
With newrow
.Range(1) = TextBox1.Value
.Range(2) = TextBox2.Value
End With
ElseIf X < lastrow Then
X = X + 1
End If
Loop Until X > lastrow
End With

Answer 1

Scan all the rows in the table before deciding to add a new row or not, and always add Use Option Explicit to top of code to catch errors like DecriptionColumnIndex (no s).
Option Explicit
Sub Add_Click()
Const PART = "CM ECP"
Const DESCRIPTION = "Material Description"
Dim ws As Worksheet
Dim X As Integer, lastrow As Long
Dim PartColumnIndex As Integer, DescrColumnIndex As Integer
Dim PartNum As String, MaterialDescription As String
Dim tbl As ListObject, bExists As Boolean
Set ws = Sheet1
Set tbl = ws.ListObjects("Master")
With tbl
PartColumnIndex = .ListColumns(PART).Index
DescrColumnIndex = .ListColumns(DESCRIPTION).Index
PartNum = Trim(TextBox1.Value)
MaterialDescription = Trim(TextBox2.Value)
' search
With .DataBodyRange
lastrow = .Rows.Count
For X = 1 To lastrow
If .Cells(X, PartColumnIndex).Value = PartNum Then
bExists = True
Exit For
End If
Next
End With
' result
If bExists = True Then
MsgBox "Part Number `" & PartNum & "` already exists on Row " & X & vbLf & _
"Please try again or return to main screen.", vbExclamation
Else
With .ListRows.Add
.Range(, PartColumnIndex) = PartNum
.Range(, DescrColumnIndex) = MaterialDescription
End With
MsgBox "Part Number `" & PartNum & "` added", vbInformation
End If
End With
End Sub