Raise an error when duplicate tool names are registered

This raises an exception when duplicate tool names are registered,
instead of silently overwriting tools. Tool names are required to be
unique within a server, so this behavior aligns with the MCP specification.

> Tool names SHOULD be unique within a server.

https://modelcontextprotocol.io/specification/2025-11-25/server/tools#tool-names

Validation for tool names could be made more strictly spec-compliant in a separate effort.

Fixes #197

lib/mcp/server.rb

@@ -5,6 +5,15 @@ require_relative "instrumentation"
 require_relative "methods"
 
 module MCP
+  class ToolNotUnique < StandardError
+    def initialize(duplicated_tool_names)
+      super(<<~MESSAGE)
+        Tool names should be unique. Use `tool_name` to assign unique names to:
+        #{duplicated_tool_names.join(", ")}
+      MESSAGE
+    end
+  end
+
   class Server
     DEFAULT_VERSION = "0.1.0"
 
@@ -53,6 +62,7 @@ module MCP
       @title = title
       @version = version
       @instructions = instructions
+      @tool_names = tools.map(&:name_value)
       @tools = tools.to_h { |t| [t.name_value, t] }
       @prompts = prompts.to_h { |p| [p.name_value, p] }
       @resources = resources
@@ -101,7 +111,10 @@ module MCP
 
     def define_tool(name: nil, title: nil, description: nil, input_schema: nil, annotations: nil, meta: nil, &block)
       tool = Tool.define(name:, title:, description:, input_schema:, annotations:, meta:, &block)
-      @tools[tool.name_value] = tool
+      tool_name = tool.name_value
+
+      @tool_names << tool_name
+      @tools[tool_name] = tool
 
       validate!
     end
@@ -176,6 +189,8 @@ module MCP
     private
 
     def validate!
+      validate_tool_name!
+
       # NOTE: The draft protocol version is the next version after 2025-11-25.
       if @configuration.protocol_version <= "2025-06-18"
         if server_info.key?(:description)
@@ -216,6 +231,12 @@ module MCP
       end
     end
 
+    def validate_tool_name!
+      duplicated_tool_names = @tool_names.tally.filter_map { |name, count| name if count >= 2 }
+
+      raise ToolNotUnique, duplicated_tool_names unless duplicated_tool_names.empty?
+    end
+
     def handle_request(request, method)
       handler = @handlers[method]
       unless handler