How to check if a scheduled Rules action set has already been scheduled? I'm using the Rules Scheduler (sub-module of the Rules module) to schedule an email reminder based on a date field set in the content type. So a user can request a reminder email on a specific date, to update that content.

However, if I invoke the Rules Action on node update, then I end up with lots of emails scheduled for that date.

How can I make sure that the rule is only scheduled once for the given date?
The answer to this is simple. 

In the action of the rule that calls the schedule component evaluation, in the Identifier section, set a unique value, such as:
reminder-[node:title]-[node:nid]

This value will be a unique identifier of the scheduled task. Scheduled tasks with the same ID will simply be replaced. Problem solved.