fix: avoid triggering on_kernel_startup cleanup multiple times

Author: maartenbreddels

solara/server/kernel_context.py

@@ -150,6 +150,9 @@ def __exit__(self, *args):
         current_context[key] = local.kernel_context_stack.pop()
 
     def close(self):
+        if self.closed_event.is_set():
+            logger.error("Tried to close a kernel context that is already closed: %s", self.id)
+            return
         logger.info("Shut down virtual kernel: %s", self.id)
         with self:
             for f in reversed(self._on_close_callbacks):