You didn't say if the ComboBox was on a UserForm or Worksheet, but the
coding would be similar. The following is for a UserForm.

Private Sub UserForm_Activate()
Dim iEnd As Long
Dim iRow As Long
Dim NoDupes As Collection
Dim ws As Worksheet

On Error Resume Next
Set ws = Worksheets("Sheet1")
iEnd = ws.Cells(65536, "C").End(xlUp).Row
Set NoDupes = New Collection
For iRow = 1 To iEnd
NoDupes.Add ws.Cells(iRow, "C"), ws.Cells(iRow, "C")
If Err.Number = 0 Then
ComboBox1.AddItem ws.Cells(iRow, "C")
Else
Err.Clear
End If
Next iRow
End Sub

Hth,
Merjet