masternode genkey is similar to generating a normal keypair (i.e. getnewaddress, dumpprivkey) which you can also run multiple times and get multiple results. The different is the masternode keypair has a different purpose: can't spend, can only control the masternode. It's confusing because you don't see the other half of the key pair when you run masternode genkey. If someone gets the masternode key, all they can do it take control of the node (voting, payouts, etc), but the collateral is safe. Presumably, you'd notice the hijack and generate a new masternode keypair if that happened, which is why you are allowed to generate as many as you need.