Code to Validate for Unique Values

I am using the following within my spreadsheet (Excel Version 14.0.7015.1000) 32 Bit. It adds a value to the next empty cell within column C. I would appreciate if someone could show me how to modify this such that it throws a text error if the value is not unique to column C and then the offending cell is selected. I tried using the built-in data validation but it seems to be bypassed when using the code below.

My excel vba level is currently "Dangerously Ignorant" so I'm happy to clarify anything you need to help.

Private Sub TextBox1_Change()

'Debug.Print "Outside " & IsActive

'To prevent recursion
If Not IsActive And TextBox1.Text <> "" Then
IsActive = True
Application.OnTime Now + TimeValue("00:00:03"), "Module2.CopyToCell"
End If

End Sub

_______

Private Sub UserForm_Initialize()
IsActive = False
TextBox1.SetFocus
End Sub


_________

Dim IsActive As Boolean

Sub CopyToCell()

Dim Ws1 As Worksheet
Dim LastCellInRng As Range

Set Ws1 = Worksheets("Main")
Set LastCellInRng = Ws1.Range("C" & Ws1.Cells.Rows.Count).End(xlUp)
LastCellInRng.Offset(1, 0).Value = UserForm2.TextBox1.Text

UserForm2.TextBox1.Text = ""
UserForm2.TextBox1.SetFocus
IsActive = False

End Sub

Well, I found one other issue. Maybe its how I incorporated parts of both your codes. The message comes up if the value is a good or nonunique one. I coped the code below.

Sub CopyToCell()

With Worksheets("Main")
If Application.CountIf(.Range("C:C"), UserForm2.TextBox1.Text) = 0 Then
.Range("C" & .Rows.Count).End(xlUp)(2).Value = UserForm2.TextBox1.Text
Else
MsgBox "That Item Already Exists"
End If
End With

UserForm2.TextBox1.Text = ""
UserForm2.TextBox1.SetFocus
IsActive = False

End Sub

Thank you so much for replying.

I do have command buttons but please realize that for this particular part the entry is via barcode scanner so when I scan I just want it to update the next cell with that value.

Your second code is working great with the addition of the else statement in your first code. I get the error message but I seem to need to click OK 3 times to clear the message box. Any ideas?

That took care of it. Thank you so much for your help.