If Columns(3).Find(what:=x.Offset(, -2), lookat:=xlWhole) Is Nothing Then

Sub example2()
    Dim x As Range
    Dim wb0 As Workbook
    Dim wl0 As Worksheet
    Application.ScreenUpdating = False
    Set wb0 = ThisWorkbook
    Set wl0 = wb0.ActiveSheet
    wb0.Activate
    With Workbooks.Open("C:\Учет .xlsx")
        For Each x In wl0.UsedRange.Cells
            If LCase(x.Value) Like "*окна*" Then
                rk = .Worksheets("Лист1").Cells(Rows.Count, 3).End(xlUp).Row
                If Not .Worksheets("Лист1").Columns(3).Find(x.Offset(, -2), , , 1) Is Nothing Then
                    .Worksheets("Лист1").Cells(rk + 1, 3).Value = x.Offset(, -2)
                End If
            End If
        Next x
'.Close True
    End With   
End Sub