Duplicating ACKs and Window Sizes
In simple words: ack is the way of the receiver to tell you "i got data until some num now send me from this num",now if a packet is lost it means the reciver will not get its data.

If i understood your question right then we can see the acks in the 12th and 13th packets are identical,the reason is because not data was sent between them to the reciver

Now why the window size is bigger?its simply because the receiver reads the data in a certain speed and the buffer now has more space in it

So th reason they have same acks its because no data was sent to the receiver
And the reason why the window size is bigger is because the receiver reads from the buffer