Inbound to SJU, don't think you'd be Duty Free from your outbound origin or airline onboard.

We just connected thru SJU from Tortolla, so I can share that return though...
Stopped in Duty Free after we cleared SJU TSA (love TSA Precheck btw)
Bought 2 1ltr bottles of Stoli - 2 for $28.00
They put it in a little carrying case with a plastic draw string "seal". The clerk then showed me how to pull back the Flap on the top to expose the bottle top!

Bingo...two miniatures @$7.00 -- or a liter @$14.00. Can you do the math?
Party hardy in row 22!
Plus it went well with sprites and pizza in our CLT connection, and carried the takeout drink on to the plane.

Slept most of the way on the last flight. Can't image why!?

As you have cleared US SECURITY out of SJU, you can carry-On to the connections, as you don't have to go thru TSA again.

Skal!