New scheduled rule knocks out existing scheduled rule I have a rule set with an active rule to send mail to a user. This is triggered by a workflow being marked done. 

This seems to work just fine until more than one, completely seperate, workflow is marked done.

When I view the scheduling rule set page that shows what's in queue for scheduling I can see the mail that is about to go out from the workflow I marked as done. Now when I go ahead and mark another workflow as done it seems to replace the email that was about to go out to another user from the previous workflow being marked done with the most recent one.

Any ideas as to why I cannot have more than one scheduled rule set in queue at a time?

D6, rules module up to date at v1.5
Make sure the Identifier for the scheduled task is unique. Adding the node ID of the node that completed the workflow could work to ensure the Identifier field is unique.

A new task will replace an existing task with the same Identifier. This happens because the Identifier value is the key each task is stored with in cron.

Check out this free video Introducing Rules Scheduler for more tips.