fix: ensure a local instance is not registered multiple times

closes #4203

src/Lean/Meta/Basic.lean

@@ -1446,7 +1446,9 @@ def withLocalInstancesImp (decls : List LocalDecl) (k : MetaM α) : MetaM α :=
   for decl in decls do
     unless decl.isImplementationDetail do
       if let some className ← isClass? decl.type then
-        localInsts := localInsts.push { className, fvar := decl.toExpr }
+        -- Ensure we don't add the same local instance multiple times.
+        unless localInsts.any fun localInst => localInst.fvar.fvarId! == decl.fvarId do
+          localInsts := localInsts.push { className, fvar := decl.toExpr }
   if localInsts.size == size then
     k
   else

tests/lean/run/4203.lean

@@ -0,0 +1,28 @@
+class Fintype (α : Type u) where
+
+class Preorder (α : Type u) extends LT α, LE α where
+  lt := fun a b => a ≤ b ∧ ¬b ≤ a
+
+structure Mappish (dIn dOut : Type u) [Fintype dIn] [Fintype dOut] where
+  k : Nat
+
+variable {dIn dOut dOut₂ : Type} [Fintype dIn] [Fintype dOut] [Fintype dOut₂]
+
+def IsGood [DecidableEq dOut] [DecidableEq dOut₂] (Λ : Mappish dIn dOut) (Λ₂ : Mappish dIn dOut₂) : Prop :=
+  ∃ (D : Mappish dOut (dOut₂)), D.k = Λ.k + Λ₂.k
+
+/--
+error: failed to synthesize
+  Fintype v
+use `set_option diagnostics true` to get diagnostic information
+-/
+#guard_msgs in
+def MappishOrder [DecidableEq dIn] : Preorder
+    (Σ dOut : Sigma (fun t ↦ Fintype t × DecidableEq t), let fin := dOut.snd.1; Mappish dIn dOut.fst) where
+  le Λ₁ Λ₂ := by
+    let u := Λ₁.fst.fst;
+    let v := Λ₂.fst.fst;
+    let ⟨w,x⟩ := Λ₁.fst.snd;
+    let ⟨y,z⟩ := Λ₂.fst.snd;
+    clear y;
+    exact @IsGood dIn v u _ _ _ _ _ Λ₂.snd Λ₁.snd