yea, you can actually add the same node to the same engine, the NodeID is unique key (I think) which is auto assigned when added so never an issue.
I have to manage that process with a policy as I also have a handful of people adding devices.
It would be nice if the MENU -> NODES -> FIND search feature checked the entire DB and reported all findings including which Polling Engine the device is on (we have 2 addit PEs).
We find we have duplicates every now and then. I setup an SQL report to run which shows me any duplicate entries - a quick check every couple of weeks keeps things straight.
Edit: Slightly more helpful if I also post the SQL we use...
SELECT Caption, COUNT(Caption) AS NumberOfDups
GROUP BY Caption
HAVING (COUNT(Caption) > 1)