diff --git a/telethon/client/telegrambaseclient.py b/telethon/client/telegrambaseclient.py
index 3fc2bcb9..e0fea3d2 100644
--- a/telethon/client/telegrambaseclient.py
+++ b/telethon/client/telegrambaseclient.py
@@ -1,5 +1,6 @@
 import abc
 import asyncio
+import collections
 import logging
 import platform
 import time
@@ -322,8 +323,9 @@ class TelegramBaseClient(abc.ABC):
 
         # Some further state for subclasses
         self._event_builders = []
-        self._conversations = {}
-        self._ids_in_conversations = {}  # chat_id: count
+
+        # {chat_id: {Conversation}}
+        self._conversations = collections.defaultdict(set)
 
         # Default parse mode
         self._parse_mode = markdown
diff --git a/telethon/client/updates.py b/telethon/client/updates.py
index c6df1546..69333a8a 100644
--- a/telethon/client/updates.py
+++ b/telethon/client/updates.py
@@ -382,8 +382,8 @@ class UpdateMethods(UserMethods):
                 await self._get_difference(update, channel_id, pts_date)
 
         built = EventBuilderDict(self, update)
-        if self._conversations:
-            for conv in self._conversations.values():
+        for conv_set in self._conversations.values():
+            for conv in conv_set:
                 ev = built[events.NewMessage]
                 if ev:
                     conv._on_new_message(ev)
diff --git a/telethon/tl/custom/conversation.py b/telethon/tl/custom/conversation.py
index f4fa234d..343bf6ec 100644
--- a/telethon/tl/custom/conversation.py
+++ b/telethon/tl/custom/conversation.py
@@ -394,12 +394,11 @@ class Conversation(ChatGetter):
 
         # Make sure we're the only conversation in this chat if it's exclusive
         chat_id = utils.get_peer_id(self._chat_peer)
-        count = self._client._ids_in_conversations.get(chat_id, 0)
-        if self._exclusive and count:
+        conv_set = self._client._conversations[chat_id]
+        if self._exclusive and conv_set:
             raise errors.AlreadyInConversationError()
 
-        self._client._ids_in_conversations[chat_id] = count + 1
-        self._client._conversations[self._id] = self
+        conv_set.add(self)
 
         self._last_outgoing = 0
         self._last_incoming = 0
@@ -426,14 +425,24 @@ class Conversation(ChatGetter):
         """
         self._cancel_all()
 
+    async def cancel_all(self):
+        """
+        Calls `cancel` on *all* conversations in this chat.
+
+        Note that you should ``await`` this method, since it's meant to be
+        used outside of a context manager, and it needs to resolve the chat.
+        """
+        chat_id = await self._client.get_peer_id(self._input_chat)
+        for conv in self._client._conversations[chat_id]:
+            conv.cancel()
+
     async def __aexit__(self, exc_type, exc_val, exc_tb):
         chat_id = utils.get_peer_id(self._chat_peer)
-        if self._client._ids_in_conversations[chat_id] == 1:
-            del self._client._ids_in_conversations[chat_id]
-        else:
-            self._client._ids_in_conversations[chat_id] -= 1
+        conv_set = self._client._conversations[chat_id]
+        conv_set.discard(self)
+        if not conv_set:
+            del self._client._conversations[chat_id]
 
-        del self._client._conversations[self._id]
         self._cancel_all()
 
     __enter__ = helpers._sync_enter