text: Title: [ASK] SAR with timedecay_formula= False won't work
  Body: ### Description
  How should I run SAR if I don't want to implement the time decay formula in SAR?
  
  My model is constructed as:
  
  model = SAR(
        col_user="userID",
        col_item="itemID",
        col_rating="rating",
        col_timestamp="timestamp",
        similarity_type= similarity_type,
        timedecay_formula = False
  )
  
  and when fitting the model, it shows:
  
  237         if df[select_columns].duplicated().any():
  --> 238             raise ValueError("There should not be duplicates in the dataframe")
      239 
      240         # generate continuous indices if this hasn't been done
  
  ValueError: There should not be duplicates in the dataframe
  
  Plese advise, thanks!
  
  ### Other Comments
  