Assign new unique key

I am trying to make a memory game in next js. I am taking a list of letters of the cards and making 2 of each then randomizing them. This part is work. However, because I made a copy of all the object in the array, I no longer have a unique key. I tried two was of adding a key but both failed.

try 1.

const lettersX2 = letters.concat(letters)
    .sort(() => Math.random() - 0.5)
    .map((card) => ({...card, order: Math.random() }));

the did not seem to do any thing.

Try 2.

const lettersX2 = letters.concat(letters)
    .sort(() => Math.random() - 0.5)
    .concat(cards.order = Math.random());

this added a new object at the end of the.

How can a add a new unique key to my array?