Remove duplicate form combobox - excel

Question

I am working on a sheet that have day to day sales data. I need to summaries the data between a specific date. for this I want to use a user form with 2 combo box (I have never worked with user forms & controls ever before). I added the items into combo box by using below codes -
Private Sub UserForm_Initialize()
ComboBox1.RowSource = "A2:A6724"
ComboBox2.RowSource = "A2:A6724"
End Sub
this worked fine. But here is a problem that it is repeating the same items many time as there are many transactions in same date in the sheet.
To solve this issue I search help in internet & found a procedure, I modify that and used in my code. that's working correctly but it also has a little problem that as I click on a date from drop down list of combo box it changes the date format (i.e. if I select 10/12/2016 it shows 12-oct-2016 but it should be 10-dec-2016)
here is the code I modify actually I don't know what it does but I think is will work for me-
Private Sub UserForm_Initialize()
'ComboBox1.RowSource = "A2:A6724"
'ComboBox2.RowSource = "A2:A6724"
Dim Coll As Collection, cell As Range, LastRow As Long
Dim blnUnsorted As Boolean, i As Integer, temp As Variant
Dim SourceSheet As Worksheet
Set SourceSheet = Worksheets("Sheet1")
LastRow = SourceSheet.Cells(Rows.Count, 1).End(xlUp).Row
On Error Resume Next
Set Coll = New Collection
With ComboBox1
.Clear
For Each cell In SourceSheet.Range("A2:A" & LastRow)
If Len(cell.Value) <> 0 Then
Err.Clear
Coll.Add cell.Text, cell.Text
If Err.Number = 0 Then .AddItem cell.Text
End If
Next cell
End With
Set SourceSheet = Worksheets("Sheet1")
LastRow = SourceSheet.Cells(Rows.Count, 1).End(xlUp).Row
On Error Resume Next
Set Coll = New Collection
With ComboBox2
.Clear
For Each cell In SourceSheet.Range("A2:A" & LastRow)
If Len(cell.Value) <> 0 Then
Err.Clear
Coll.Add cell.Text, cell.Text
If Err.Number = 0 Then .AddItem cell.Text
End If
Next cell
End With
Set Coll = Nothing
Set SourceSheet = Nothing
End Sub
I will be greatly Thankful for any help.

Answer 1

Try following code, that use a dictionary.
Public dU1 As Object, cU1 As Variant, iU1 As Long, lrU As Long
Private Sub UserForm_Initialize()
Dim i As Integer
Set dU1 = CreateObject("Scripting.Dictionary")
lrU = Worksheets("Sheet1").Cells(Rows.Count, 1).End(xlUp).Row
cU1 = Worksheets("Sheet1").Range("A2:A" & lrU) 'Starts in second row. First row left for titles
For iU1 = 1 To UBound(cU1, 1)
dU1(cU1(iU1, 1)) = 1
Next iU1
'now dU1 has unique values from column A
For i = 0 To dU1.Count - 1
ComboBox1.AddItem dU1.Keys()(i) 'Load Combobox1 with unique values from Column A
Next
End Sub
Private Sub ComboBox1_Change()
Dim lLastRow As Long
Dim i As Integer
ComboBox2.Clear
For i = 0 To dU1.Count - 1
If CDate(ComboBox1.Value) < CDate(dU1.Keys()(i)) Then
ComboBox2.AddItem dU1.Keys()(i) 'Load Combobox2
End If
Next
End Sub