marking duplicates

[comp23]

i have lots of data in column a and i want to check for duplicates. what code would i need to use so that everytime a duplicate is found it turns the cell red

[HSV]

Will this work for you.

Sub hsv()
Dim cl As Range
 For Each cl In Columns(1).SpecialCells(2)
  If WorksheetFunction.CountIf(Columns(1), cl) > 1 Then cl.Interior.ColorIndex = 3
 Next cl
End Sub

[zbor]

Use in Conditional formattin: =COUNTIF($A$1:$A$1000, $A1)>1