When a meeting is copied, Copy: is prefixed to the subject of meetings when they are copied within a calendar or to another calendar. This 'feature' was added to Outlook beginning with Outlook 2007 SP2.

Copy: was added so users would know how the meeting ended up on the calendar and why it wasn't updating. There is no way to avoid it in Outlook, except by not copying the meetings. Note that this applies to all copied meetings. Any time you make a copy of a meeting by copying it to a new date, copying it to a second calendar in your mailbox, or export it to a pst, Copy: is added to the subject.

You can edit the subject line or use VBA to remove it.

The Infobar on meeting will say if the meeting was copied. Copy: won't be added back to the subject if you copy it again.

If you want to remove the 'Invitation' prefix from meetings originating from Gmail calendars, see Remove Invitation Prefix from Gmail Meetings for a ready-to-use macro solution.

Solutions

The following methods did not add the Copy: prefix to subject lines in my tests:

1. If you are using a pst, copy the pst file then move the meetings from the pst copy to the second Outlook. Moving the meetings will preserve their ability to accept or send updates.
2. If you are using Exchange server, use Outlook 2003 to export the calendar.
3. Create a new pst. Right click on the calendar and choose Copy. Select the new pst as the destination. Outlook will copy the items then complain it can't copy the folder. Open the pst in the second Outlook and move the calendar items.

Use VBA to remove Copy:

Below is a VBA script you can use to cycle through every appointment item in the selected calendar and remove the Copy: prefix. Works in Outlook 2007 and 2010. (Also works in older versions, if you need to mass-edit the subject line.)

October 7 2018: Updated the macro to use the Replace function.

Sub RemoveCopy()  
Dim myolApp As Outlook.Application  
Dim calendar As MAPIFolder  
Dim aItem As Object  

Set myolApp = CreateObject("Outlook.Application")  
Set calendar = myolApp.ActiveExplorer.CurrentFolder  

Dim iItemsUpdated As Integer  
Dim strTemp As String  

iItemsUpdated = 0  
For Each Item In calendar.Items  
    If Mid(Item.Subject, 1, 6) = "Copy: " Then  
      strTemp = Replace(Item.Subject, "Copy: ", "")  
      Item.Subject = strTemp  
      iItemsUpdated = iItemsUpdated + 1  
    End If  
    Item.Save  
Next Item  

MsgBox iItemsUpdated & " of " & calendar.Items.Count & " Meetings Updated"  

End Sub

How to use VBA

Select the calendar that needs fixed.

1. Press Alt+F11 to open the VBA editor. (or Tools, Macro, Visual Basic Editor)
2. Expand Project1 then double click on ThisOutlookSession to open the code window.
3. Copy the code above (click in the text, Ctrl+A to select all, Ctrl+C to copy)
4. Paste it into the code window then Save.
5. Press the Run button.

To run the code later, press Alt+F8 (or Tools, Macro, Macros), select the RemoveCopy macro and press Run.