Static analyzer should error on reassigning implicit with same key in same scope

Problem

Currently the static analyzer allows reassigning an implicit variable with the same key in the same scope:

let scope = ImplicitScope()
defer { scope.end() }

@Implicit(\.id) var id = 10

// This should be an error - reassigning same key in same scope
@Implicit(\.id) var newId = 999

This is confusing and likely a bug.

Expected Behavior

The static analyzer should emit an error when the same implicit key is assigned twice in the same scope.

Correct Pattern for Overriding

To override implicit values, use a nested scope in a separate function:

func entry() {
  let scope = ImplicitScope()
  defer { scope.end() }

  @Implicit(\.id) var id = 10
  
  doWork(scope)
}

func doWork(_ scope: ImplicitScope) {
  let scope = scope.nested()
  defer { scope.end() }

  @Implicit(\.id) var overriddenId = 999
  // ...
}

No file or test entry point is named. Start by locating the Swift static analyzer logic that validates @Implicit keys, then reproduce the two declarations in the issue's same-scope example. Done means the analyzer emits an error for duplicate keys in one scope while still allowing the nested-scope pattern.