What does it mean?
You can request the server to duplicate or triplicate a packet that the server sends to your client. And you can tell the client to send every packet twice or 3 times as well. In case you have packet loss, this could reduce the packet loss significantly.

How does it work?
Server packet duplication:
To ask the server to send a packet twice, simply type in the client console. 1 means that the server will send 1 extra packet for each packet. will tell the server to send 2 extra packet for each packet.
You can enable some debug messages in the client with . The packets coming from the server are the ones with the "<--" mark, and your client's packets are marked with the "-->" mark. You should see 2 (or more) lines with "<--" marks if you enabled server packet duplication.

Client packet duplication:
To ask the client to send a packet twice, simply type in the client console. Just like for server packet duplication, 1 here will mean that your client is sending 1 extra packet for each packet.
Unfortunately the debug message that you activate with will not show duplicate (or triplicate) packets sent by your client. There is no (easy) way to track or visualize this. The server will drop anyhow any duplicated packet, so actually there will be only 1 packet that reaches the server.
This feature was added to ezquake around February 2021, so you will need a ezquake-3.6 at least.

Keep in mind that this feature only helps in reducing packet loss. Has nothing to do with ping.
Also keep in mind that if you request the server to send packet duplication, it will only send multiple packets to your client, and to noone else on the server.