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Copy and paste the below code into the files making sure you name them correct!
You can make a folder inside your interface/addons folder and name it anything you want.

SilentWhisper.lua
Code:
 
local VERSION="0.1.0" 
function SilentWhisper_OnLoad() 
this:RegisterEvent("CHAT_MSG_WHISPER"); 
end 
function SilentWhisper_OnEvent(event) 
if strsub(event,1,16) == "CHAT_MSG_WHISPER" then 
local msg = arg1; 
local plr = arg2; 
Whisper(msg) 
end 
end 
function Whisper(Message) 
local rname = "ChangeMe"; 
SendChatMessage(Message , "WHISPER", "Common", rname); 
end
SilentWhisper.toc
Code:
## Interface: 333000 
## Title: Silent Whisper 
## Author: Falkor 
## Version: 0.1.0 
## Notes: Redirects whispers from your alts to your main 
## DefaultState: Enabled 
## LoadOnDemand: 0 
SilentWhisper.xml
SilentWhisper.xml
Code:
<Ui xmlns="http://www.blizzard.com/wow/ui/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.blizzard.com/wow/ui/ C:\Projects\WoW\Bin\Interface\FrameXML\UI.xsd"> 
<Script file="SilentWhisper.lua" /> 
<Frame name="SilentFrame" parent="UIParent" frameStrata="HIGH" toplevel="true" hidden="true"> 
<Scripts> 
<OnLoad> 
SilentWhisper_OnLoad(); 
</OnLoad> 
<OnEvent> 
SilentWhisper_OnEvent(event); 
</OnEvent> 
</Scripts> 
</Frame> 
</Ui>