lazyLoad throws a warning when showing more than one item per slide

### What is the error?
Console shows 'Warning: Encountered two children with the same key, ...' when switching pages.

### When does it occur?

1. `lazyLoad: true` **AND** `rows: >1`
2. `lazyLoad: true` **AND** `slidesPerRow: >1`

### When does it NOT occur?

1. `lazyLoad: false`
2. `lazyLoad: true` **AND** (`rows: 1` **AND** `slidesPerRow: 1` )

### Sandbox
https://codesandbox.io/s/mqz2n3o8yy