Sam:
I'm interested in hearing other people chiming in on this as well.
Right now I'm setting up an engagement engine that has a few streams mapped to stages in the buyers' journey. When someone shows enough interest in the next area of the journey (awareness, research, consideration, decision), we push them to the next stream. The point being, we send different types of content based on the stream, and some emails are used in multiple streams (to gauge their point in the buying cycle).
For example, a new lead might get nurtured with a "decision" piece in the first stream (Awareness). If they show activity in "decision," we send them to the appropriate stream, and they get more "decision" pieces. In the Decision stream, they'll get lots of "decision" pieces.
The short version of this to summarize: we have all of our emails housed in the engagement program separated into appropriate folders. Some emails get used in multiple streams. As long as the emails are housed in the same convenient location (to make sure the same email is added to separate streams, and not cloned), leads won't get the same email twice.
Best,
Edward UnthankSEO/Web Specialist
Yesler