Multiple lookup solution needed...

Hey all, I've run into a stumper, well for me.

I am currently running the SingleCellExtract VBA (below) to present all column 2 entries for matches to column 1. My issue lies that every 2 column list has thousands of entries and so running the extract take hours and hours. Does anyone know 1. a better solution to look through the data and relay what I'm looking for and 2. that solution could be "living" so when new entries are added everything "refreshes".

This is the code I'm running:

Function SingleCellExtract(LookupValue As String, LookupRange As Range, ColumnNumber As Integer, Char As String)
    Dim I As Long
    Dim xRet As String
Application.ScreenUpdating = False

    For I = 1 To LookupRange.Columns(1).Cells.Count
        If LookupRange.Cells(I, 1) = LookupValue Then
            If xRet = "" Then
                xRet = LookupRange.Cells(I, ColumnNumber) & Char
            Else
                xRet = xRet & "" & LookupRange.Cells(I, ColumnNumber) & Char
            End If
        End If
    Next
    SingleCellExtract = Left(xRet, Len(xRet) - 1)
Application.ScreenUpdating = True
End Function

example of Data:
Cell A - Cell B - Cell C (were formula/code is running)
12345 - 9.15.16 - 9.15.16, 10.2.16
67890 - 9.16.16 - 9.16.16, 10.3.16
12345 - 10.2.16 - 9.15.16, 10.2.16
67890 - 10.3.16 - 9.16.16, 10.3.16

I appreciate any help with speeding this up, the first project took 12 hours to complete column C.
Thanks! Nick.