Finally, I have found the proper way to do it with react. Please note that it does not concern ag-grid but it only react rendeting mechanism.

• addItem() BEFORE, not working (with duplicates we want to avoid):

  const addItem = useCallback((item: any) => {
    const exist = selectedItems.findIndex(
      (existingItem) => existingItem.name === item.name
    );
    if (exist === -1) {
      setSelectedItems((items) => items.concat(item));
    }
  }, []);

• addItem() AFTER, working properly (without duplicates, and (I think) React oriented) :

 const addItem = useCallback((item: any) => {
      setSelectedItems((items) => {
      const exist = items.findIndex(
         (existingItem) => existingItem.name === item.name
      );
      if (exist === -1) {
        return items.concat(item);
      }
     return items;
     });
 }, []);

Codesandbox updated