How two devices can be assigned the same IP when both are online?

This is correct.  If devices request an IP address using the DHCP protocol, the DHCP process is supposed to respond with an IP address that is not currently on the network, which it would discover by doing an Address Resolution Protocol (ARP) broadcast.  However, devices which do not function as they should can interfere with the DHCP process.  It could have been that one of those two devices ignored the ARP broadcast and did not respond, "Hey, that is the IP address I am using!"

One of the things that annoys me about Netgear's implementation of DHCP is that it does not enforce the LAN Setup definitions.  Suppose, for example, that I connect a new device to the network and it gets assigned 192.168.1.AAA  I decide that "this is a camera, and I put all my cameras in a group of sequential IP addresses", so I use the LAN Setup table to assign 192.168.1.44 to that camera.  Every time the camera DHCP lease runs out, it requests a renewal of 192.168.1.AAA and the Orbi says, "sure. keep using that."  HEY, says the crabby grandpa.  "I told you to assign 192.18.1.44 to that camera!!!" Grrrr.

There are only two ways to get the camera to switch IP addresses:

• Power off the camera. Set another device to a static IP address of 192.168.1.AAA and then power up the camera, or
• Reboot the entire Orbi system, which will cause every device on the network to perform a new DHCP request, and "voila" the camera will get the 192.168.1.44 that I told Orbi to give it.

Either method is annoying.

The key to investigating problems is to be able to replicate them at will. There very well may be a bug in the DHCP mechanism for the 950 product.  If this keeps happening, please document it and (if you are within the magic "90 days of complimentary support") open a case with Netgear.