Clarify double instrumentation issue (#13751)

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## DESCRIBE YOUR PR
A clarifying sentence to explain how to avoid double instrumentation of
Celery beat tasks in Python.

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Author: antonpirker

docs/platforms/python/integrations/celery/crons.mdx

@@ -42,6 +42,8 @@ Next, initialize Sentry. Where to do this depends on how you run beat:
 
 Make sure to also set `monitor_beat_tasks=True` in `CeleryIntegration`.
 
+If you have set `monitor_beat_tasks=True` make sure that you do not do manual task monitoring using the `@sentry_sdk.monitor` decorator, because this will lead to double instrumentation.
+
 In addition to capturing errors, you can monitor interactions between multiple services or applications by [enabling tracing](/concepts/key-terms/tracing/). You can also collect and analyze performance profiles from real users with [profiling](/product/explore/profiling/).
 
 Select which Sentry features you'd like to install in addition to Error Monitoring to get the corresponding installation and configuration instructions below.