Highlighting Duplicate Values In Each Row of a Multiple Row Worksheet

A Selection is a Range made visible on the sheet. Application.ScreenUpdating = False makes the Selection invisible. What's the point? Stop thinking in terms of selecting anything. Instead, focus on defining Range objects and manipulating them.

The task is to create a Range object for each row.

Dim Rng As Range
Dim NumRows As Long
Dim R As Long

NumRows = Range("B2:BY20").Rows.Count
For R = 2 To NumRows
    Set Rng = Range(Cells(R, 2), Cells(R, Columns("BY").Column))
    Debug.Print Rng.Address
Next R

The above code will set a range and print its address in the Immediate Window. Of course, you might also apply conditional formatting to it.

The above code is also an example of how to post code on this forum. To enable the effect of copying the code easily please enclose the entire code in one set of code brackets. Had you done that (and you might still do, if you like) I would have commented your code.

Edit 25 Oct 2018  =================================

The code below combines your original code for setting the CF (slightly reformatted) but applied to a dynamic Range object instead of a static Selection object.

Sub ApplyCF()
    ' 25 Oct 2018
    
    Dim Ws As Worksheet
    Dim Rng As Range                    ' variable range
    Dim Rl As Long                      ' last used row
    Dim R As Long                       ' row counter
    Dim Idx As Integer                  ' FormatConditions index
    
    Set Ws = ActiveSheet                ' better: Set Ws = Worksheets("SheetName")
                                        ' to prevent code being run on the wrong sheet
    Application.ScreenUpdating = False
    Rl = Ws.Cells(Ws.Rows.Count, "B").End(xlUp).Row     ' last used row in column B
    
    For R = 2 To Rl                                     ' start from row 2 (change as required)
        Set Rng = Range(Ws.Cells(R, "B"), Ws.Cells(R, "BY"))
        With Rng.FormatConditions
            .AddUniqueValues
            Idx = .Count
            With .Item(Idx)
                .SetFirstPriority
                .DupeUnique = xlDuplicate
                With .Font
                    .Bold = True
                    .Italic = False
                    .ThemeColor = xlThemeColorAccent1
                    .TintAndShade = 0
                End With
                With .Interior
                    .PatternColorIndex = xlAutomatic
                    .Color = 65535
                    .TintAndShade = 0
                End With
                .StopIfTrue = False
            End With
        End With
    Next R
    
    Application.ScreenUpdating = True
End Sub