FaceTime on Macbook without iPad interference Needed to collaborate with someone on a document, editing on Macbook Pro (MBP).  Made FaceTime audio call, but then switched to video.  Top of screen claimed "call failed" but we were still talking.  Then the iPad began begging me to answer the call!  It gave up and we were still on.  iPad was not a call from someone else.

Something else disconnected us, and when I called again, this time the iPad did not interfere.

Is the first just a fluke, or must I do something to prevent the iPad from trying to hijack a call on the Macbook?  (Hopefully something other than shutting it down!)
No - there's no cross talk possible since Apple uses different encryption keys for each device. All of them receive a notification / offer to start the call and continuity would allow you to hand off a call from one device to the other.

If you're distracted or worried, you can sign out of FaceTime / Messages on the device you want to be sure isn't bugging you, but architecturally - there's no downside / benefit to having more than one device signed in other than if your network can't handle the signaling items or can't NAT the traffic correctly.