Emit error for duplicate type parameters

[fo5for]

class Test[A, A]: { x: A }
//│ Defined class Test

t = Test { x = 42 }
//│ t: Test['A | 'A0 .. 42 & 'A | 'A & 'A0, 'A | 'A0 .. 42 & 'A | 'A & 'A0] with {x: 42}

t : Test[bool, int]
//│ res: Test[bool | int .. nothing, bool | int .. nothing]

t : Test[int, bool]
//│ ╔══[ERROR] Type mismatch in type ascription:
//│ ║  l.10: 	t : Test[int, bool]
//│ ║        	^
//│ ╟── expression of type `42` does not match type `bool`
//│ ║  l.4: 	t = Test { x = 42 }
//│ ║       	               ^^
//│ ╟── but it flows into reference with expected type `Test[int, bool]`
//│ ║  l.10: 	t : Test[int, bool]
//│ ║        	^
//│ ╟── Note: constraint arises from type reference:
//│ ║  l.10: 	t : Test[int, bool]
//│ ║        	              ^^^^
//│ ╟── from record type:
//│ ║  l.1: 	class Test[A, A]: { x: A }
//│ ╙──     	                  ^^^^^^^^
//│ res: Test[bool | int .. nothing, bool | int .. nothing]

Currently the last one takes precedence, and the output contains duplicate type variables. (It breaks surprisingly few things besides ascription.)

[LPTK]

Right. BTW we have the same problem with record fields, although I'm not sure it breaks anything:

aa = {a = 1; a = "oops" }
//│ aa: {a: 1, a: "oops"}

aa.a
//│ res: 1

aa: { a: 'a }
//│ res: {a: 1}

Do you think you can fix them after we've merged my PR?