Which Mac OS are you using? This is a godawful bug that's been around since Leopard (?) and still isn't fixed in Mavericks.
All you have to do is start changing the recipient in the reply message, and things go completely south.
There is a work-around though.
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try
tell application "Mail"
set quoteOriginalMessagePref to quote original message
set selectedMsg to 1st item of (get selection)
set recipientList to {name, address} of recipients of selectedMsg
set newOutgoingMessage to reply selectedMsg with opening window
delay 0.05
if quoteOriginalMessagePref = true then set quote original message to false
tell newOutgoingMessage
set address of to recipients to ""
repeat with ndx from 1 to length of (item 1 of recipientList)
make new recipient at end of to recipients with properties ¬
{name:item ndx of item 1 of recipientList, address:item ndx of item 2 of recipientList}
end repeat
end tell
if quoteOriginalMessagePref then set quote original message to true
end tell
on error e
try
tell application "Mail"
set quote original message to true
end tell
end try
e
end try
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