I am using Amazon Web Services ECS (Elastic Container Service).
My task definition contains Application + Redis + Celery and these containers are defined in task definition. Automatic scaling is set, so at the moment there are three instances with same mirrored infrastructure. However, there is a demand for a Celery Beat instance for scheduled tasks, so Celery Beat would be a great tool, since Celery is already in my infrastructure.

But here is the problem: if I add Celery Beat container together with other containers (add it to task definition), it will be mirrored and multiple instances will execute same scheduled tasks at the same moment. What would be a solution to this infrastructure problem? Should I create a seperate service?

We use single-beat to solve this problem and it works like a charm:

Single-beat is a nice little application that ensures only one
instance of your process runs across your servers.

Such as celerybeat (or some kind of daily mail sender, orphan file
cleaner etc…) needs to be running only on one server, but if that
server gets down, well, you go and start it at another server etc.

You should still set the number of desired tasks for the service to 1.