How to duplicate.
1. Your GT5 save file has a primary file and a backup file. Both can be found in the PS3 Save Data folder on the PS3's XMB. Every time you open GT5, the backup file is updated as a copy of your most recent primary save file.
2. Open GT5. Send a car from your garage or a ticket from your inventory to a friend.
3. Exit GT5. Open your PS3 Save Data folder. Check to make sure you have a primary save file, and a BACKUP save file present before you proceed. If you have both files, delete your PRIMARY save file only.
4. Open GT5. Click "YES" when the game asks if you want to load your backup save data.
5. Your backup data is restored, and you still own the car or ticket you just sent to your friend.
NOTE - Items in your gift inbox are stored online, and not in your game save data. If you want to keep a copy of a car/ticket, do not open GT5 and accept a gift car/ticket, then immediately send the new car/ticket right back out. When you reload your backup data file, your gift in your inbox will be gone and you won't have a copy of it in your garage/inventory.
Always exit the game completely back to the XMB and then reopen the game before sending any cars/tickets you are trying to duplicate, just to be safe. You need to be sure the car/ticket exists in your garage/inventory when your backup file is updated before you send the car/ticket out.
Tonight, this is what I'm going to do:
Turn on PS3
Start GT5.
Send Silverado from my garage to Gorsad or gokartman.
Exit GT5.
Delete PRIMARY GT5 save file from PS3 Save Data folder.
Start GT5.
Click "YES" to load backup data.
Drive the Silverado I just sent to Gorsad/gokartman, because I still have my original copy of it.