This is just the fastest implementation. And it takes up the least amount of memory.

Yeah, not sure why this has more "best practice"s than "clever". because it's definitely not what you should do unless it's code golf.

Crazy 1 liner right there.

added

I think you need to add test case when array doesnt have duplicates
Because this solution worked and its not really checking if array contains duplicates or not

if(arr.length === 0 || arr.length === 1) {
return false;
}
return true

this must be very slow