Unique Values in Combobox in Userform

I have a userform for users to enter data, which is then populated into the spreadsheet.
This spreadsheet is effectively a contacts database.
In column C there is a company name. As a result of having multiple contact for each company, company names are duplicated when I open the userform with a combobox initialised with:

Dim iRow As Long
Dim bNeedMore As Boolean
Dim sName As String

  iRow = 11
  bNeedMore = True
  While bNeedMore = True
    iRow = iRow + 1
    sName = Trim(Sheets("Sheet1").Cells(iRow, "C"))
    If Len(sName) > 0 Then
      CompanyComBox.AddItem sName
      CompanyComBox.List(CompanyComBox.ListCount - 1, 1) = iRow
    Else
      bNeedMore = False
    End If
  Wend

I would like to somehow alter this code to remove duplicates in column C.

Any help is appreciated.

Cheers,

nKife