diff --git a/pyproject.toml b/pyproject.toml
index 3af9438..5c46cac 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -21,6 +21,7 @@ classifiers = [
 ]
 dependencies = [
     "jsonschema",
+    "frozendict",
 ]
 
 [project.optional-dependencies]
@@ -65,4 +66,4 @@ files = [
 follow_imports = "silent"
 disallow_untyped_defs = true
 warn_return_any = true
-warn_unreachable = true
\ No newline at end of file
+warn_unreachable = true
diff --git a/src/mars_patcher/item_patcher.py b/src/mars_patcher/item_patcher.py
index 87381be..4c257bb 100644
--- a/src/mars_patcher/item_patcher.py
+++ b/src/mars_patcher/item_patcher.py
@@ -59,6 +59,7 @@ def write_items(self) -> None:
         prev_area_room = (-1, -1)
         room_tank_count = 0
         total_metroids = 0
+        item_messages_to_custom_id: dict[ItemMessages, int] = {}
         for min_loc in minor_locs:
             if min_loc.new_item == ItemType.INFANT_METROID:
                 total_metroids += 1
@@ -131,10 +132,20 @@ def write_items(self) -> None:
                     rom.write_8(item_addr + 6, min_loc.item_sprite.value)
             # Handle custom messages
             if min_loc.item_messages is not None:
-                self.write_custom_message(
-                    custom_message_id, message_table_addrs, item_addr, min_loc.item_messages, False
-                )
-                custom_message_id += 1
+                # If we already encountered the message before, just write the message id.
+                messages = min_loc.item_messages
+                if messages in item_messages_to_custom_id:
+                    rom.write_8(item_addr + 7, item_messages_to_custom_id[messages])
+                else:
+                    self.write_custom_message(
+                        custom_message_id,
+                        message_table_addrs,
+                        item_addr,
+                        min_loc.item_messages,
+                        False,
+                    )
+                    item_messages_to_custom_id[messages] = custom_message_id
+                    custom_message_id += 1
 
         # Handle major locations
         for maj_loc in self.settings.major_locs:
@@ -146,10 +157,20 @@ def write_items(self) -> None:
                 rom.write_8(addr, maj_loc.new_item.value)
                 # Handle custom messages
                 if maj_loc.item_messages is not None:
-                    self.write_custom_message(
-                        custom_message_id, message_table_addrs, addr, maj_loc.item_messages, True
-                    )
-                    custom_message_id += 1
+                    # If we already encountered the message before, just write the message id.
+                    messages = maj_loc.item_messages
+                    if messages in item_messages_to_custom_id:
+                        rom.write_8(addr + 1, item_messages_to_custom_id[messages])
+                    else:
+                        self.write_custom_message(
+                            custom_message_id,
+                            message_table_addrs,
+                            addr,
+                            maj_loc.item_messages,
+                            True,
+                        )
+                        item_messages_to_custom_id[messages] = custom_message_id
+                        custom_message_id += 1
 
         # Write total metroid count
         rom.write_8(TOTAL_METROID_COUNT_ADDR, total_metroids)
diff --git a/src/mars_patcher/locations.py b/src/mars_patcher/locations.py
index bdace40..244bf27 100644
--- a/src/mars_patcher/locations.py
+++ b/src/mars_patcher/locations.py
@@ -1,7 +1,10 @@
 from __future__ import annotations
 
 import json
-from typing import TYPE_CHECKING
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, ClassVar
+
+from frozendict import frozendict
 
 from mars_patcher.constants.items import (
     ITEM_ENUMS,
@@ -86,8 +89,12 @@ def __init__(
         self.item_messages = item_messages
 
 
+@dataclass(frozen=True)
 class ItemMessages:
-    LANG_ENUMS = {
+    item_messages: frozendict[Language, str]
+    centered: bool
+
+    LANG_ENUMS: ClassVar[dict[str, Language]] = {
         "JapaneseKanji": Language.JAPANESE_KANJI,
         "JapaneseHiragana": Language.JAPANESE_HIRAGANA,
         "English": Language.ENGLISH,
@@ -97,10 +104,6 @@ class ItemMessages:
         "Spanish": Language.SPANISH,
     }
 
-    def __init__(self, item_messages: dict[Language, str], centered: bool):
-        self.item_messages = item_messages
-        self.centered = centered
-
     @classmethod
     def from_json(cls, data: Itemmessages) -> ItemMessages:
         item_messages: dict[Language, str] = {}
@@ -108,7 +111,7 @@ def from_json(cls, data: Itemmessages) -> ItemMessages:
             lang = cls.LANG_ENUMS[lang_name]
             item_messages[lang] = message
         centered = data.get(KEY_CENTERED, True)
-        return cls(item_messages, centered)
+        return cls(frozendict(item_messages), centered)
 
 
 class LocationSettings: