From 4134a17a29e5e1cf5bd14e83ca260b472aa28a41 Mon Sep 17 00:00:00 2001
From: Sebastian Law <44045823+SebbyLaw@users.noreply.github.com>
Date: Thu, 8 Apr 2021 19:04:10 -0700
Subject: [PATCH] [commands] Raise error when a cog name is already registered

---
 discord/ext/commands/bot.py | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/discord/ext/commands/bot.py b/discord/ext/commands/bot.py
index dabe14398f4f..4ab5ee35021e 100644
--- a/discord/ext/commands/bot.py
+++ b/discord/ext/commands/bot.py
@@ -489,15 +489,25 @@ def decorator(func):
 
     # cogs
 
-    def add_cog(self, cog):
+    def add_cog(self, cog: Cog, *, override: bool = False) -> None:
         """Adds a "cog" to the bot.
 
         A cog is a class that has its own event listeners and commands.
 
+        .. versionchanged:: 2.0
+
+            :exc:`.ClientException` is raised when a cog with the same name
+            is already loaded.
+
         Parameters
         -----------
         cog: :class:`.Cog`
             The cog to register to the bot.
+        override: :class:`bool`
+            If a previously loaded cog with the same name should be ejected
+            instead of raising an error.
+
+            .. versionadded:: 2.0
 
         Raises
         -------
@@ -505,13 +515,23 @@ def add_cog(self, cog):
             The cog does not inherit from :class:`.Cog`.
         CommandError
             An error happened during loading.
+        .ClientException
+            A cog with the same name is already loaded.
         """
 
         if not isinstance(cog, Cog):
             raise TypeError('cogs must derive from Cog')
 
+        cog_name = cog.__cog_name__
+        existing = self.__cogs.get(cog_name)
+
+        if existing is not None:
+            if not override:
+                raise discord.ClientException(f'Cog named {cog_name!r} already loaded')
+            self.remove_cog(cog_name)
+
         cog = cog._inject(self)
-        self.__cogs[cog.__cog_name__] = cog
+        self.__cogs[cog_name] = cog
 
     def get_cog(self, name):
         """Gets the cog instance requested.