How to copy the column A to M from a row in sheet1 to sheet2 if it doesn't already exist - excel

Question

I would like to select some rows by putting an X in the column L, then copy selected row (Only column A to M) to the next free row in sheet2.
Free row mean there is nothing in the column A to M since there is content in the next column already filled.
The copy shouldn't erase the content already existing after column M.
The row can't be added if it's already in the sheet2 and to test this, I have an unique ID for the row in column M.
Some of the column of the row that should be copied are sometimes empty.
Part of what I tried :
Sub GAtoList()
Dim xRg As Range
Dim xCell As Range
Dim A As Long
Dim B As Long
Dim L As Long
A = Worksheets("knxexport").Range("d" & Worksheets("knxexport").Rows.Count)
B = Worksheets("Sheet2").UsedRange.Rows.Count
If B = 1 Then
If Application.WorksheetFunction.CountA(Worksheets("Sheet2").UsedRange) = 0 Then B = 0
End If
Set xRg = Worksheets("knxexport").Range("L1:L" & A)
Application.ScreenUpdating = False
For L = 1 To xRg.Count
If CStr(xRg(L).Value) = "X" Then
xRg(L).EntireRow.Copy Destination:=Worksheets("Sheet2").Range("A" & B + 1)
B = B + 1
Cells(L, B).EntireRow.Interior.ColorIndex = 4
End If
Next
'Erase the X that select the row I want to copy
Worksheets("knxexport").Columns(12).ClearContents
Worksheets("Sheet2").Columns(12).ClearContents
Application.ScreenUpdating = True
End Sub
Column D is never empty so I use it to check the end of the source sheet
knxexport sheet where I take data
sheet2 where I want to copy them

Answer 1

Please, test the next code:
Sub GAtoList()
Dim sh As Worksheet, shDest As Worksheet, lastRL As Long, LastRM As Long
Dim strSearch As String, rngM As Range, arrCopy, cellF As Range, rngL As Range, cellFAddress As String, i As Long, mtch
strSearch = "X"
Set sh = 'Worksheets("knxexport") 'the sheet to copy from
Set shDest = 'Worksheets("Sheet2") 'the sheet to copy to
shDest.Range("M:M").NumberFormat = "#" 'format the M:M column as text
lastRL = sh.Range("L" & sh.rows.count).End(xlUp).row
Set rngL = sh.Range("L2:L" & lastRL) 'the range to search for "X"
Set cellF = rngL.Find(what:=strSearch, After:=sh.Range("L2"), LookIn:=xlValues, Lookat:=xlWhole, MatchCase:=False)
If Not cellF Is Nothing Then 'If at least an "X" string has been found:
cellFAddress = cellF.Address 'memorize its (first) address
Do
LastRM = shDest.Range("M" & shDest.rows.count).End(xlUp).row 'last row in M:M
If LastRM > 1 Then 'if there already are IDs:
Set rngM = shDest.Range("M2:M" & LastRM)
mtch = Application.match(sh.cells(cellF.row, "M").Value, rngM, 0)
If IsError(mtch) Then 'no ID found
shDest.Range("A" & LastRM + 1 & ":" & "M" & LastRM + 1).Value = _
sh.Range(sh.Range("A" & cellF.row), sh.Range("M" & cellF.row)).Value
Else
Debug.Print sh.cells(cellF.row, "M").Value & " already existing..." 'warn in case of ID existence...
End If
Else
'copy in the second row
shDest.Range("A2:M2").Value = _
sh.Range(sh.Range("A" & cellF.row), sh.Range("M" & cellF.row)).Value
End If
Set cellF = rngL.FindNext(cellF)
Loop While cellF.Address <> cellFAddress 'exit to avoid restarting loop from the memorized address
Else
MsgBox strSearch & " could not be found in ""L:"" column...": Exit Sub
End If
End Sub