Why deleted Active Directory objects sometimes Reappear?

ne common cause of resurrected objects is offline domain controllers. If a domain controller (DC) is offline for more than60 days, it will have a copy of the deleted object, which no other DC has. Since no other DC will have a record of the deleted object, it will be re-distributed across the domain. This problem can also occur if a DC is restored from a backup that is more than60 days old. The best way to prevent this is to make daily or weekly AD backups and never return a DC to the network before it has been updated.

For detail of answer of this please read following article

http://eddiejackson.net/web_documents/The_Tips_and_Tricks_Guide_to_Active_Directory_Troubleshooting.pdf