Warn on duplicate keys (feature request)

[Darkle]

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Hi, I was having issues with vue-virtual-scroller with items not showing up properly. I was trying a lot of different things to figure out what was going wrong, then eventually i realised there was a bug in my code and i was sending items with duplicate keys in to the list.

It would be really helpful if vue-virtual-scroller could fire off a message to the console warning that there were duplicate keys detected. What do you think?