Disallow duplicate free variables in defs (#11965)

Makes the following a syntax error:

```crystal
def foo(x : T) forall T, T # Error: duplicated free variable name: T
end
```

Currently the two `T`s are indistinguishable, but if something like #11908 is eventually implemented, the same free variable might be bounded in one place but not another (`T <= Int32, T`), or it might have two different bounds (`T <= Int32, T <= String`). This PR ensures every name to the right of `forall` defines a distinct free variable.

spec/compiler/parser/parser_spec.cr

@@ -382,6 +382,7 @@ module Crystal
     it_parses "def foo(x : U) : Int32 forall T, U; end", Def.new("foo", args: [Arg.new("x", restriction: "U".path)], return_type: "Int32".path, free_vars: %w(T U))
     assert_syntax_error "def foo(x : U) forall; end"
     assert_syntax_error "def foo(x : U) forall U,; end"
+    assert_syntax_error "def foo(x : U) forall U, U; end", "duplicated free variable name: U"
 
     it_parses "foo", "foo".call
     it_parses "foo()", "foo".call

src/compiler/crystal/syntax/parser.cr

@@ -3685,7 +3685,9 @@ module Crystal
       free_vars = [] of String
       while true
         check :CONST
-        free_vars << @token.value.to_s
+        free_var = @token.value.to_s
+        raise "duplicated free variable name: #{free_var}", @token if free_vars.includes?(free_var)
+        free_vars << free_var
 
         next_token_skip_space
         if @token.type.op_comma?