The patches are downloaded only once and they are stored in the RMM Central Server. The default location where the patches gets stored is <RMM_Central_Server_Home>/ManagementCentral/webapps/Desktopentral/store and is configurable. However, you need to copy the previously downloaded patches to the new location manually, else the patches will get downloaded to the new location again.
You can change the local patch store to a different location using the "Change Patch Store Directory" link available in the Downloaded Patches view.