I have some JSX that is giving me the error:
`Warning: Each child in a list should have a unique "key" prop`
which I've run across before, but this time I can't see where the problem is. The markup is as follows:

```
    return isLoading ? (
      <div className="App">
        <div className="spinner"></div>
        <div className="App loading">
          <p><i>loading...</i></p>
        </div>
      </div>
    )
    : hasError ? (
      <div className="App loading-error">
        &#x26A0; There is a network issue: Please try again later
      </div>
    )
    :
    (
      <div className="App" key={`op - ${Math.floor(Math.random() * 110)}`}>

        <>
        {shelves.map(s => {
          return (
           books[s].map((b,i) => {
            return (
                <>
                <h2>{((shelvesN.filter(o => o.value === s)))[0].label}</h2>
                <div key={`${s}-${i}`}>
                    <Link to={`/book/${b.id}`}>
                    <img src={b.imageLinks.thumbnail} alt={b.title}></img>
                    </Link>
                    <Select className="sel-x"
                            placeholder="Choose a bookshelf..."
                            value={""}
                            options={shelvesN}
                            onChange={opt => changeShelf(opt.value,b.id)}
                    />
                </div>
                </>
            )
          })
          )
        })}
        </>
      </div>
    )

```

I'd appreciate any help. Thanks.

k.