Fig.: it refers to the attempt of refreshing/restarting, after some time, something that has already gone wrong, knowing that probably it will go wrong again. It refers particularly to love situations

 Please don't try to arrange a meeting between us. It's all over and done with

È inutile che cerchiate di farmelo incontrare. Non sarebbero che cavoli riscaldati

After breaking up for third time, the two “lovebirds” got back together, but everybody knew it wouldn't last because it was just old hat

Dopo essersi lasciati per la terza volta, i due fidanzati sono ritornati assieme, ma a detta di tutti non sarebbe durata perché era minestra riscaldata

The expression dates from the late 19th century, and it probably refers to the fact that hats go out of fashion before they actually wear out