You're getting 8 messages because the script tells mIRC to send one message for each active connection that the bot is on, and the bot has 8 active connections.
While you might not have 8 connections, it's the number of connections that the client running the script that matters.
In order to send the message to one specific connection, you would need to know either the connection ID for that connection, or the relevant connection number. Note: The connection ID is not likely to be the same as the connection number.
To get the connection number, you're going to have to figure out which connections were made before the one that you're wanting to use. To my knowledge, mIRC doesn't have an identifier for this information.
To get the connection ID, you can change to that connection, then type //echo -a $cid