Grabbed as-is from https://github.com/elixir-lang/elixir/commit/54516c5684daa3af7f15477c77fae39e8d0c6482 which was done for 1.15.x

From 54516c5684daa3af7f15477c77fae39e8d0c6482 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Valim?= <jose.valim@dashbit.co>
Date: Sun, 1 Oct 2023 15:46:17 +0200
Subject: [PATCH] Address tests on Erlang/OTP 26.1, closes #12975

---
 lib/ex_unit/lib/ex_unit/callbacks.ex         |  9 +-----
 lib/ex_unit/test/ex_unit/formatter_test.exs  | 31 +++++++++-----------
 lib/ex_unit/test/ex_unit/supervised_test.exs | 11 ++-----
 3 files changed, 18 insertions(+), 33 deletions(-)

diff --git a/lib/ex_unit/lib/ex_unit/callbacks.ex b/lib/ex_unit/lib/ex_unit/callbacks.ex
index 060d207516a..2dbe9bd0e4a 100644
--- a/lib/ex_unit/lib/ex_unit/callbacks.ex
+++ b/lib/ex_unit/lib/ex_unit/callbacks.ex
@@ -549,14 +549,7 @@ defmodule ExUnit.Callbacks do
       end
 
     child_spec = Supervisor.child_spec(child_spec_or_module, opts)
-
-    case Supervisor.start_child(sup, child_spec) do
-      {:error, {:already_started, _pid}} ->
-        {:error, {:duplicate_child_name, child_spec.id}}
-
-      other ->
-        other
-    end
+    Supervisor.start_child(sup, child_spec)
   end
 
   @doc """
diff --git a/lib/ex_unit/test/ex_unit/formatter_test.exs b/lib/ex_unit/test/ex_unit/formatter_test.exs
index afb66a52436..bb9642fbcc4 100644
--- a/lib/ex_unit/test/ex_unit/formatter_test.exs
+++ b/lib/ex_unit/test/ex_unit/formatter_test.exs
@@ -491,28 +491,25 @@ defmodule ExUnit.FormatterTest do
   test "inspect failure" do
     failure = [{:error, catch_assertion(assert :will_fail == %BadInspect{}), []}]
 
-    message = ~S'''
-      got FunctionClauseError with message:
-
-          """
-          no function clause matching in Inspect.ExUnit.FormatterTest.BadInspect.inspect/2
-          """
-
-      while inspecting:
-
-          %{__struct__: ExUnit.FormatterTest.BadInspect, key: 0}
-
-      Stacktrace:
-    '''
-
-    assert format_test_failure(test(), failure, 1, 80, &formatter/2) =~ """
+    assert format_test_failure(test(), failure, 1, 80, &formatter/2) =~ ~s'''
              1) world (Hello)
                 test/ex_unit/formatter_test.exs:1
                 Assertion with == failed
                 code:  assert :will_fail == %BadInspect{}
                 left:  :will_fail
-                right: #Inspect.Error<\n#{message}\
-           """
+                right: #Inspect.Error<
+             got FunctionClauseError with message:
+
+                 """
+                 no function clause matching in Inspect.ExUnit.FormatterTest.BadInspect.inspect/2
+                 """
+
+             while inspecting:
+
+                 #{inspect(%BadInspect{}, structs: false)}
+
+             Stacktrace:
+           '''
   end
 
   defmodule BadMessage do
diff --git a/lib/ex_unit/test/ex_unit/supervised_test.exs b/lib/ex_unit/test/ex_unit/supervised_test.exs
index 577ae6b9676..a02dfe3c0c5 100644
--- a/lib/ex_unit/test/ex_unit/supervised_test.exs
+++ b/lib/ex_unit/test/ex_unit/supervised_test.exs
@@ -73,19 +73,14 @@ defmodule ExUnit.SupervisedTest do
 
   test "starts a supervised process with ID checks" do
     {:ok, pid} = start_supervised({MyAgent, 0})
+    assert is_pid(pid)
 
-    assert {:error, {:duplicate_child_name, ExUnit.SupervisedTest.MyAgent}} =
-             start_supervised({MyAgent, 0})
-
-    assert {:error, {{:already_started, ^pid}, _}} = start_supervised({MyAgent, 0}, id: :another)
+    assert {:error, _} = start_supervised({MyAgent, 0})
+    assert {:error, _} = start_supervised({MyAgent, 0}, id: :another)
 
     assert_raise RuntimeError, ~r"Reason: bad child specification", fn ->
       start_supervised!(%{id: 1, start: :oops})
     end
-
-    assert_raise RuntimeError, ~r"Reason: already started", fn ->
-      start_supervised!({MyAgent, 0}, id: :another)
-    end
   end
 
   test "stops a supervised process" do