What does "A duplicate notice on this case was mailed" mean?

[bumblebee]

My I-140 case status reads "A duplicate notice on this case was mailed on ....". What does this mean ??

 

[tyzh]

It means "A notice on this case was mailed on ....".

 

[bumblebee]

But this case said "Your case was approved...." before. Now does it mean the approval notice has been sent twice? Or does it mean the case has been withdrawn?

 

[tyzh]

Then it means:

   Your case was approved....
+ A notice on this case was mailed
--------------------------------------------------------