Remove duplicate entries in Set.fromList/Dict.fromList

[jfmengels]

If some of the keys are the same, then we can remove some of them (potentially telling the user there's a problem with their setup).

Dict.fromList [ ( 1, a ), ( 2, b ), ( 3, c ), ( 1, d ) ]
-->
Dict.fromList [ ( 2, b ), ( 3, c ), ( 1, d ) ]

Dict.fromList [ ( 1, a ), ( 2, b ), ( 3, c ) ]
  |> Dict.insert 1 d
-->
Dict.fromList [ ( 2, b ), ( 3, c ) ]
  |> Dict.insert 1 d
--> Or alternatively put it directly in the list, probably nicer
Dict.fromList [ ( 2, b ), ( 3, c ), ( 1, d ) ]


Set.fromList [ 1, 2, 3, 1 ]
-->
Set.fromList [ 2, 3, 1 ]
--> or remove the later one 🤷 
Set.fromList [ 1, 2, 3 ]

Set.fromList [ 1, 2, 3 ]
  |> Set.insert 1
-->
Set.fromList [ 1, 2, 3 ]

[lue-bird]

We can also implement these checks for unknown key expressions like

Set.fromList [ a, a ]
--> Set.fromList [ a ]

NaNs do not follow this rule

Set.fromList [ 0/0, 0/0 ]
--> Set.fromList [NaN,NaN]

so we should only do it if expectNaN is configured False.

---

I think I'll implement some of those

[lue-bird]

Another simplification we can do is

Dict.fromList [ a, anything, a ]
--> Dict.fromList [ anything, a ]

(again, doesn't work when expecting NaN)