Generate New Attribute

Hello,

I want to generate an attribute [AttNew]. The values of the new attribute should be according to the following rule:

• If ID xy only exist one time
  
• --> For ID xy Take the value of the Att1 for the new Attribute [AttNew]
  

• If there are several identical IDs,
• -> take all the values of Att 1 of the identical IDs
  
• --> and include them as a new attribute value for the generated attribute [2]
  

Is there an operator which does that? Or can anyone help how to describe the function?

Thank you very much!