Calling chidlList.add to add an element that's already in the childList throws "out of bounds" error

[rodrigojmr]

We're now gradually bumping Lightning to the latest version and came across this when bumping to 2.5.0, and it should still occur in latest.

Caused by #314

This makes it so we have have to check if the item exists in the childList before calling add every time, at least for certain uses.

Say we have an item that may be attached and detached from different childLists, as it's something like a reusable visual focus element. When we've added that item to the childList before and try to add it again, the check using currentIndex and index (in this context, the length of the array) will be incorrect. Say currentIndex is 1 and the length of the childList is 2, so that's the index.

    addAt(item, index) {
        if (index >= 0 && index <= this._items.length) {
            let currentIndex = this._items.indexOf(item);
            // Never true when being called by `add`
            if (currentIndex === index) {
                return item;
            }
          ......
            if (currentIndex != -1) {
                this.setAt(item, index);
          ......
}

    setAt(item, index) {
        // Never true, throws out of bounds error
        if (index >= 0 && index < this._items.length) {

This worked in 2.4.0, even if it triggered this whole flow:

            let currentIndex = this._items.indexOf(item);
            if (currentIndex != -1) {
                if (currentIndex !== index) {
                    const fromIndex = currentIndex;
                    if (fromIndex !== index) {
                        this._items.splice(fromIndex, 1);
                        this._items.splice(index, 0, item);
                        this.onMove(item, fromIndex, index);
                    }
                }

Surely this could be optimized? Or is this the responsability of the application developers, and if so, shouldn't this be documented?