Download app update once, install twice/thrice?
The "Caching Server" is one of the features of OS X Server. Whenever one of your computers (iOS or OS X) downloads an update it is cached on the server.  This link has a simple description: Caching Server

This is the official solution that Apple provide.  It does mean another level of complexity to managing your network and only you you will be able to decide whether you want to follow the OS X Server path. From my point of view, I would not unless I intended to use some of the other features of OS X Server (e.g. centralised Time Machine). The server Mac can continue to be used for desktop activity.