diff --git a/src/mcp/server/fastmcp/server.py b/src/mcp/server/fastmcp/server.py
index 924baaa9b..c536a96c3 100644
--- a/src/mcp/server/fastmcp/server.py
+++ b/src/mcp/server/fastmcp/server.py
@@ -418,14 +418,15 @@ async def async_tool(x: int, context: Context) -> str:
             )
 
         def decorator(fn: AnyFunction) -> AnyFunction:
-            self.add_tool(
-                fn,
-                name=name,
-                title=title,
-                description=description,
-                annotations=annotations,
-                structured_output=structured_output,
-            )
+            if not self._tool_manager.has_tool(fn.__name__):
+                self.add_tool(
+                    fn,
+                    name=name,
+                    title=title,
+                    description=description,
+                    annotations=annotations,
+                    structured_output=structured_output,
+                )
             return fn
 
         return decorator
diff --git a/src/mcp/server/fastmcp/tools/tool_manager.py b/src/mcp/server/fastmcp/tools/tool_manager.py
index bfa8b2382..174e9da96 100644
--- a/src/mcp/server/fastmcp/tools/tool_manager.py
+++ b/src/mcp/server/fastmcp/tools/tool_manager.py
@@ -38,6 +38,10 @@ def get_tool(self, name: str) -> Tool | None:
         """Get tool by name."""
         return self._tools.get(name)
 
+    def has_tool(self, name: str) -> bool:
+        """Check if a tool exists."""
+        return name in self._tools
+
     def list_tools(self) -> list[Tool]:
         """List all registered tools."""
         return list(self._tools.values())