diff --git a/CHANGES.md b/CHANGES.md
index 864f0a54410..6f4e6ed5bca 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -7,6 +7,7 @@
 - Add new `--workers` parameter (#2514)
 - Fixed feature detection for positional-only arguments in lambdas (#2532)
 - Bumped typed-ast version minimum to 1.4.3 for 3.10 compatiblity (#2519)
+- Run formatter twice internally to ensure stability (#2549)
 
 ### _Blackd_
 
diff --git a/src/black/__init__.py b/src/black/__init__.py
index fdbaf040d64..6d8bf920811 100644
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -879,7 +879,7 @@ def format_stdin_to_stdout(
 
 
 def check_stability_and_equivalence(
-    src_contents: str, dst_contents: str, *, mode: Mode
+    src_contents: str, dst_contents_pass1: str, dst_contents_pass2: str, *, mode: Mode
 ) -> None:
     """Perform stability and equivalence checks.
 
@@ -887,16 +887,14 @@ def check_stability_and_equivalence(
     equivalent, or if a second pass of the formatter would format the
     content differently.
     """
-    assert_equivalent(src_contents, dst_contents)
+    assert_equivalent(src_contents, dst_contents_pass1)
 
-    # Forced second pass to work around optional trailing commas (becoming
+    # Require second pass to work around optional trailing commas (becoming
     # forced trailing commas on pass 2) interacting differently with optional
     # parentheses.  Admittedly ugly.
-    dst_contents_pass2 = format_str(dst_contents, mode=mode)
-    if dst_contents != dst_contents_pass2:
-        dst_contents = dst_contents_pass2
-        assert_equivalent(src_contents, dst_contents, pass_num=2)
-        assert_stable(src_contents, dst_contents, mode=mode)
+    if dst_contents_pass1 != dst_contents_pass2:
+        assert_equivalent(src_contents, dst_contents_pass2, pass_num=2)
+        assert_stable(src_contents, dst_contents_pass2, mode=mode)
     # Note: no need to explicitly call `assert_stable` if `dst_contents` was
     # the same as `dst_contents_pass2`.
 
@@ -914,13 +912,18 @@ def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileCo
     if mode.is_ipynb:
         dst_contents = format_ipynb_string(src_contents, fast=fast, mode=mode)
     else:
-        dst_contents = format_str(src_contents, mode=mode)
+        dst_contents_pass1 = format_str(src_contents, mode=mode)
+        dst_contents = format_str(dst_contents_pass1, mode=mode)
+
     if src_contents == dst_contents:
         raise NothingChanged
 
     if not fast and not mode.is_ipynb:
         # Jupyter notebooks will already have been checked above.
-        check_stability_and_equivalence(src_contents, dst_contents, mode=mode)
+        check_stability_and_equivalence(
+            src_contents, dst_contents_pass1, dst_contents, mode=mode
+        )
+
     return dst_contents
 
 
@@ -967,9 +970,12 @@ def format_cell(src: str, *, fast: bool, mode: Mode) -> str:
         masked_src, replacements = mask_cell(src_without_trailing_semicolon)
     except SyntaxError:
         raise NothingChanged from None
-    masked_dst = format_str(masked_src, mode=mode)
+    masked_dst_pass1 = format_str(masked_src, mode=mode)
+    masked_dst = format_str(masked_dst_pass1, mode=mode)
     if not fast:
-        check_stability_and_equivalence(masked_src, masked_dst, mode=mode)
+        check_stability_and_equivalence(
+            masked_src, masked_dst_pass1, masked_dst, mode=mode
+        )
     dst_without_trailing_semicolon = unmask_cell(masked_dst, replacements)
     dst = put_trailing_semicolon_back(
         dst_without_trailing_semicolon, has_trailing_semicolon
diff --git a/tests/data/function_trailing_comma.py b/tests/data/function_trailing_comma.py
index 02078219e82..956840789ec 100644
--- a/tests/data/function_trailing_comma.py
+++ b/tests/data/function_trailing_comma.py
@@ -15,6 +15,10 @@ def f(a:int=1,):
     }["a"]
     if a == {"a": 1,"b": 2,"c": 3,"d": 4,"e": 5,"f": 6,"g": 7,"h": 8,}["a"]:
         pass
+    if {"a": 1,"b": 2,"c": 3,"d": 4,"e": 5,"f": 6,"g": 7,"h": 8,}["a"] == a:
+        pass
+    assert a == call({"a": 1,"b": 2,"c": 3,"d": 4,"e": 5,"f": 6,"g": 7,"h": 8,}["a"])
+    assert call({"a": 1,"b": 2,"c": 3,"d": 4,"e": 5,"f": 6,"g": 7,"h": 8,}["a"]) == a
 
 def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> Set[
     "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
@@ -100,6 +104,44 @@ def f(
         "h": 8,
     }["a"]:
         pass
+    if {
+        "a": 1,
+        "b": 2,
+        "c": 3,
+        "d": 4,
+        "e": 5,
+        "f": 6,
+        "g": 7,
+        "h": 8,
+    }["a"] == a:
+        pass
+    assert a == call(
+        {
+            "a": 1,
+            "b": 2,
+            "c": 3,
+            "d": 4,
+            "e": 5,
+            "f": 6,
+            "g": 7,
+            "h": 8,
+        }["a"]
+    )
+    assert (
+        call(
+            {
+                "a": 1,
+                "b": 2,
+                "c": 3,
+                "d": 4,
+                "e": 5,
+                "f": 6,
+                "g": 7,
+                "h": 8,
+            }["a"]
+        )
+        == a
+    )
 
 
 def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> Set[
diff --git a/tests/util.py b/tests/util.py
index 84e98bb0fbd..11a4f7f5dae 100644
--- a/tests/util.py
+++ b/tests/util.py
@@ -9,7 +9,7 @@
 import black
 from black.debug import DebugVisitor
 from black.mode import TargetVersion
-from black.output import err, out
+from black.output import err, out, diff
 
 THIS_DIR = Path(__file__).parent
 DATA_DIR = THIS_DIR / "data"
@@ -47,6 +47,9 @@ def _assert_format_equal(expected: str, actual: str) -> None:
         except Exception as ve:
             err(str(ve))
 
+    if actual != expected:
+        out(diff(expected, actual, "expected", "actual"))
+
     assert actual == expected