Remove duplicates from array using VBA

Question

Assume I have a block of data in Excel 2010, 100 rows by 3 columns.

Column C contains some duplicates, say it starts off as

1, 1, 1, 2, 3, 4,

Answer 1

An improvement on @RBILLC and @radoslav006 answers, this version searches the array with the duplicates removed for existing values so it searchs less values to find a duplicate.

Public Function RemoveDuplicatesFromArray(sourceArray As Variant)
    Dim duplicateFound As Boolean
    Dim arrayIndex As Integer, i As Integer, j As Integer
    Dim deduplicatedArray() As Variant
    
    arrayIndex = -1
    deduplicatedArray = Array(1)

    For i = LBound(sourceArray) To UBound(sourceArray)
        duplicateFound = False

        For j = LBound(deduplicatedArray) To UBound(deduplicatedArray)
            If sourceArray(i) = deduplicatedArray(j) Then
                duplicateFound = True
                Exit For
            End If
        Next j

        If duplicateFound = False Then
            arrayIndex = arrayIndex + 1
            ReDim Preserve deduplicatedArray(arrayIndex)
            deduplicatedArray(arrayIndex) = sourceArray(i)
        End If
    Next i

    RemoveDuplicatesFromArray = deduplicatedArray
End Function

Answer 2

Remove duplicates (plus related row items) from array

As OP wanted a VBA solution close to RemoveDuplicates, I demonstrate an array approach using a ►dictionary to get not the unique items per se (dict.keys), but the related row indices of first occurrencies (dict.items).

These are used to retain the whole row data via procedure LeaveUniques profiting from the advanced possibilities of the ►Application.Index() function - c.f. Some peculiarities of the the Application.Index function

Example Call

Sub ExampleCall()
'[0]define range and assign data to 1-based 2-dim datafield
    With Sheet1                   ' << reference to your project's sheet Code(Name)
        Dim lastRow: lastRow = .Cells(.Rows.Count, "C").End(xlUp).Row
        Dim rng:  Set rng = .Range("C2:E" & lastRow)
    End With
    Dim data: data = rng        ' assign data to 2-dim datafield
'[1]get uniques (column 1) and remove duplicate rows
    LeaveUniques data           ' << call procedure LeaveUniques (c.f. RemoveDuplicates)
'[2]overwrite original range
    rng.Clear
    rng.Resize(UBound(data), UBound(data, 2)) = data
End Sub

Procedure LeaveUniques

Sub LeaveUniques(ByRef data As Variant, Optional ByVal colNum As Long = 1)
'Purpose: procedure removes duplicates of given column number in entire array
    data = Application.Index(data, uniqueRowIndices(data, colNum), nColIndices(UBound(data, 2)))
End Sub

Help functions to LeaveUniques

Function uniqueRowIndices(data, Optional ByVal colNum As Long = 1)
'Purpose: return data index numbers referring to uniques
'a) set late bound dictionary to memory
    Dim dict As Object
    Set dict = CreateObject("Scripting.Dictionary")
'b) slice e.g. first data column (colNum = 1)
    Dim colData
    colData = Application.Index(data, 0, colNum)
'c) fill dictionary with uniques referring to first occurencies
    Dim i As Long
    For i = 1 To UBound(colData)
        If Not dict.exists(dict(colData(i, 1))) Then dict(colData(i, 1)) = i
    Next
'd) return 2-dim array of valid unique 1-based index numbers
    uniqueRowIndices = Application.Transpose(dict.items)
End Function

Function nColIndices(ByVal n As Long)
'Purpose: return "flat" array of n column indices, e.g. for n = 3 ~> Array(1, 2, 3)
    nColIndices = Application.Transpose(Evaluate("row(1:" & n & ")"))
End Function