How to find repeated character from given dynamic string

str="i am busy at work in my company"
str=Replace(str," ","")
For i=1 To Len(str)-1
 If Len(str)=0 Then
  Exit for
 End If
 k=Mid(str,1,1)
 If InStr(Right(str,Len(str)-1),k)>0 Then
  str2=str2&k
  str=Replace(str,k,"")
  Else
  str=Replace(str,k,"")
 End If
Next


MsgBox str2