Don't apply self-referential replacements twice

CHANGELOG.md

@@ -7,6 +7,7 @@
 - Fix type annotation issues identified by mypy.
 - Run CI against pull requests.
 - Fix package build warnings.
+- Fix custom replacements being applied twice when a replacement contains its own search value (e.g. `['a', 'aa']` no longer yields `aaaa`).
 
 ## 8.0.4
 

slugify/slugify.py

@@ -185,6 +185,11 @@ def slugify(
     # finalize user-specific replacements
     if replacements:
         for old, new in replacements:
+            # skip self-referential rules (``old`` present in ``new``); these were
+            # already applied in the pre-process pass and re-applying them here
+            # would grow the slug on every run (e.g. ['a', 'aa'] -> 'aaaa').
+            if old and old in new:
+                continue
             text = text.replace(old, new)
 
     # smart truncate if requested

test.py

@@ -237,6 +237,15 @@ def test_replacements_german_umlaut_custom(self):
         r = slugify(txt, replacements=[['Ü', 'UE'], ['ü', 'ue']])
         self.assertEqual(r, "ueber-ueber-german-umlaut")
 
+    def test_replacements_not_applied_twice(self):
+        # A replacement whose value contains its own key must not be applied
+        # twice (pre + finalize pass), which used to grow the slug.
+        r = slugify('a', replacements=[['a', 'aa']])
+        self.assertEqual(r, "aa")
+
+        r = slugify('hello', replacements=[['l', 'll']])
+        self.assertEqual(r, "hellllo")
+
     def test_pre_translation(self):
         self.assertEqual(PRE_TRANSLATIONS, [('Ю', 'U'), ('Щ', 'Sch'), ('У', 'Y'), ('Х', 'H'), ('Я', 'Ya'), ('Ё', 'E'), ('ё', 'e'), ('я', 'ya'), ('х', 'h'), ('у', 'y'), ('щ', 'sch'), ('ю', 'u'), ('Ü', 'Ue'), ('Ö', 'Oe'), ('Ä', 'Ae'), ('ä', 'ae'), ('ö', 'oe'), ('ü', 'ue'), ('Ϋ́', 'Y'), ('Ϋ', 'Y'), ('Ύ', 'Y'), ('Υ', 'Y'), ('Χ', 'Ch'), ('χ', 'ch'), ('Ξ', 'X'), ('ϒ', 'Y'), ('υ', 'y'), ('ύ', 'y'), ('ϋ', 'y'), ('ΰ', 'y')])