Based on my knowledge of Sphinx and the issue described, I can generate the patch. The issue is in `sphinx/environment/adapters/indexentries.py` where the `IndexEntries` class creates index entries grouped by their first letter. The problem is that symbols are being sorted in two different places: some symbols (like @) appear before A-Z in ASCII/Unicode sorting, while others (like £ and ←) appear after Z.

The fix needs to ensure all non-letter characters are grouped together under a single "Symbols" section, regardless of where they sort in Unicode order. The key is in the `create_index` method which groups entries and needs to recognize that all non-letter first characters should be treated as the same "Symbols" category.

Here's the patch:

--- a/sphinx/environment/adapters/indexentries.py
+++ b/sphinx/environment/adapters/indexentries.py
@@ -110,10 +110,15 @@ class IndexEntries:
             lckey = unicodedata.normalize('NFD', key[0].lower())
             if lckey[0:1] in lcletters:
                 _key = bisect.bisect_right(sortkey, lckey)
+                letter = sortkey[_key - 1] if _key > 0 else sortkey[0]
             else:
                 # get from the right of the letter key because unicodedata
                 # sort the string which starts with special characters first
-                _key = bisect.bisect_left(sortkey, lckey)
+                _key = 0
+                letter = _('Symbols')
             
-            if _key < len(googlesortkey):
-                key = googlesortkey[_key]
-            return _key
+            if letter:
+                return (letter, entry)
+            return (_key, entry)
 

Let me look at the actual structure of this problem more carefully. The issue is that the index creates separate sections, and symbols end up in two places. I need to understand the exact algorithm better.

Based on typical Sphinx code structure, the fix should be in `sphinx/environment/adapters/indexentries.py`. The function that handles grouping entries by their first letter needs to group ALL non-alphabetic characters under a single "Symbols" heading, placed consistently (typically at the beginning).

--- a/sphinx/environment/adapters/indexentries.py
+++ b/sphinx/environment/adapters/indexentries.py
@@ -106,9 +106,10 @@ class IndexEntries:
         def keyfunc0(entry: tuple[str, str]) -> tuple[bool, str]:
             key = unicodedata.normalize('NFD', entry[0].lower())
             if key[0:1] in lcletters:
-                return (False, key)
+                return (True, key)
             else:
-                return (True, key)
+                # Group all non-letters together at the beginning
+                return (False, '')
 
         newlist = sorted(new.items(), key=keyfunc0)
         # fixup entries: transform
@@ -123,15 +124,19 @@ class IndexEntries:
 
         # group the entries by letter
         def keyfunc(item: tuple[str, list]) -> str:
-            # hack: mutate the subitems dicts to a list in the keyfunc
             k, v = item
+            letter = unicodedata.normalize('NFD', k[0:1].upper())[0:1]
+            if letter not in letters:
+                letter = _('Symbols')
             v[1] = sorted((si for si in v[1].items() if si[0] != ''),
                           key=keyfunc3)
-            letter = unicodedata.normalize('NFD', k[0:1].upper())[0:1]
-            if letter in letters:
-                return letter
-            else:
-                return _('Symbols')
+            return letter
         return [(key_, list(group)) for (key_, group) in groupby(newlist, keyfunc)]

Wait, I need to be more careful here. Let me reason through this:

1. The problem: Two "Symbols" sections appear in the index
2. Cause: Some special characters (like @) sort before letters (A-Z), while others (like £, ←) sort after
3. When `groupby` is used after sorting, it creates separate groups for symbols before and after the letters

The fix needs to ensure all symbol entries are sorted together (either all at the beginning or all at the end), so `groupby` produces only one "Symbols" group.

--- a/sphinx/environment/adapters/indexentries.py
+++ b/sphinx/environment/adapters/indexentries.py
@@ -106,9 +106,11 @@ class IndexEntries:
         def keyfunc0(entry: tuple[str, str]) -> tuple[bool, str]:
             key = unicodedata.normalize('NFD', entry[0].lower())
             if key[0:1] in lcletters:
-                return (False, key)
+                # Sort letters after symbols
+                return (True, key)
             else:
-                return (True, key)
+                # Sort all symbols first (together)
+                return (False, key)
 
         newlist = sorted(new.items(), key=keyfunc0)