`rw` duplicates goals

Prerequisites

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• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
  (You can also use the settings there to switch to “Lean nightly”)

Description

The rw tactic sometimes duplicates the same MVarId in the list of goals

Context

[Broader context that the issue occurred in. If there was any prior discussion on the Lean Zulip, link it here as well.]

Steps to Reproduce

Run

theorem duplicate_goal : ∃ a : Int, a = 1 := by
  refine ⟨?_, ?_⟩
  rotate_left
  rw [eq_comm]
  -- three goals
  sorry
  -- two goals
  sorry
  -- zero goals

Expected behavior: There should be two goals after the rw

Actual behavior: The goal state after the rw is

case refine_2
⊢ 1 = ?refine_1
case refine_1
⊢ Int
case refine_1
⊢ Int

Versions

4.15.0-rc1

Additional Information

[Additional information, configuration or data that might be necessary to reproduce the issue]

Impact

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Start by running the provided duplicate_goal theorem against Lean 4.15.0-rc1 and inspect the rw [eq_comm] goal state after rotate_left. Trace the rw tactic's goal handling to find why one MVarId is emitted twice. Done means the example produces two goals after rw, without relying on Mathlib or Batteries.