EM: do not exec the same task twice

[gares]

I guess this broke when we started flattening delegated task.
There are two options:

• we we flatten we filter upfront the tasks that are already done
• we make exec aware of the cache

I did the latter, since it is more "bullet proof", although the former is in more line with the code that already filters out done tasks when preparing them.

[gares]

After more thinking, I believe this solution is OK.