Nercolytie, I've tried that, and while I can figure out timers, unit groups, and simple things like that, variables for Special Effects has me stumped. I'm going to try Juggernaut's idea, and if it fails to work I'll look into yours, perhaps there's a simpler way to do it that I've overlooked...
In what way is the DoT stacking? is it occurring through different spells or the same spell? Do you want it to be able to stack more than once from spell 1 and once from spell 2, once from both spells, or more than once from both spells?
If the Locust Swarm idea doesn't work, try to make the Eagle follow the summoner unit and make it owned by an allied computer player. The Eagle will attack any enemy it comes close to while it's following the summoner unit.
When does the error occur? Figure out the answer(s) to this question and try to fix it. Is it caused by a trigger? If it is, which one? You may need to temporarily deactivate the trigger on a public release while you work on it. I know I had to do that when I was trying to mix elementals...
I have an issue in my map. After a unit is hit, a special effect is supposed to appear, attached to the attacked unit's overhead. This special effect is supposed to disappear after 3 seconds. However, the way I have it set up now, it only deletes the last created special effect. This is an issue...
Try this, replacing Town Hall with your appropriate building and Footman with your appropriate Unit and going from there.
Also, I'm new myself. I don't know how to post pictures correctly.
I tested this on my own editor and it worked.
Are you wanting the unit or units to spawn once every 8 seconds regardless of whether or not the building is actually there, or only if the building is there?
Better yet, what exactly are you trying to achieve?