RSA signing security
Timestamps, as mentioned by user93353, are one possible answer.  The drawback is that they require synchronized clocks, which can't always be assumed.

Another possible approach to prove liveness (that is, that this isn't a replay) is for the receiver to select a random value (a "nonce"), send that to the server, and have the server sign the command concatenated with the nonce.  This also foils a replay; replays cannot work unless the receiver repeats the nonce.