diff --git a/src/command_manager.cc b/src/command_manager.cc
index fe9cc9ee..3c59dbc9 100644
--- a/src/command_manager.cc
+++ b/src/command_manager.cc
@@ -10,14 +10,21 @@
 namespace Kakoune
 {
 
-void CommandManager::register_command(const String& command_name, Command command,
+bool CommandManager::command_defined(const String& command_name) const
+{
+    return m_commands.find(command_name) != m_commands.end();
+}
+
+void CommandManager::register_command(const String& command_name,
+                                      Command command,
                                       unsigned flags,
                                       const CommandCompleter& completer)
 {
     m_commands[command_name] = CommandDescriptor { command, flags, completer };
 }
 
-void CommandManager::register_commands(const memoryview<String>& command_names, Command command,
+void CommandManager::register_commands(const memoryview<String>& command_names,
+                                       Command command,
                                        unsigned flags,
                                        const CommandCompleter& completer)
 {
diff --git a/src/command_manager.hh b/src/command_manager.hh
index 54d67b80..b3f0917e 100644
--- a/src/command_manager.hh
+++ b/src/command_manager.hh
@@ -62,6 +62,8 @@ public:
 
     Completions complete(const String& command_line, size_t cursor_pos);
 
+    bool command_defined(const String& command_name) const;
+
     void register_command(const String& command_name,
                           Command command,
                           unsigned flags = None,
diff --git a/src/commands.cc b/src/commands.cc
index 700ad552..24990f75 100644
--- a/src/commands.cc
+++ b/src/commands.cc
@@ -455,6 +455,7 @@ void define_command(const CommandParameters& params, const Context& context)
     ParametersParser parser(params,
                             { { "env-params", false },
                               { "append-params", false },
+                              { "allow-override", false },
                               { "shell-completion", true } });
 
     if (parser.positional_count() < 2)
@@ -462,6 +463,11 @@ void define_command(const CommandParameters& params, const Context& context)
 
     auto begin = parser.begin();
     const String& cmd_name = *begin;
+
+    if (CommandManager::instance().command_defined(cmd_name) and
+        not parser.has_option("allow-override"))
+        throw runtime_error("command '" + cmd_name + "' already defined");
+
     std::vector<String> cmd_params(++begin, parser.end());
     Command cmd;
     if (parser.has_option("env-params"))