Like what they say, its not doubling, the events just occur twice. What I would recommend is having a hidden JVS button somewhere. Name that button whatever like "Velocity_JVS."
Now, for the brick with the onPlayertouch events, do the events " onPlayertouch < NAMED BRICK | Velocity JVS < content start.
Ok, go to the brick "Velocity JVS," clear all events, and put in these events.
1. onContentstart < player < AddVelocity < 10 x x
2. onContentstarted < self < contentstop
This will do the action needed without making events occurring twice. Since the JVS button cannot be activated until the content has stopped, it just wont work until it has stopped. This is also good if you want buttons to only be accessed by you.
Yes, this is alot of events and there will be way to many JVS Buttons all around. Just try to stack them in some spot of nowhere and make them different colors so you know which one to event if it needs editing.