I am designing a server-client model game library/engine.

How do I, and should I even bother to handle frequent update packet possible impersonating?

In my current design anyone could copy a packet from someone else and modify it to execute any non-critical action for another client.

I am currently compressing all datagrams so that adds just a tad of security.

Edit: One way I thought about was to send a unique "key" to the verified client every x_time and then the client has to add that to all of it's update packets until a new key is sent.

Edit2: I should have mentioned that I am not concerned about whether the actions described in the packet are available to the client at the time, this is all checked by the server which I thought was obvious. I am only concerned about someone sending packets for another client.