Chapter 7 Firewall for FortiOS 5.0 : GUI & CLI - What You May Not Know : Naming Rules and Restrictions
  
Naming Rules and Restrictions
The following are the specific rules that are obeyed by the FortiGate.
Duplicate Name Issues:
A VLAN cannot have the same name as a physical interface.
An Address must not have the same name as an Address Group.
An Address or Address Group must not have the same name as a Virtual IP Address.
A Service cannot have the same name as a Service Group.
A VLAN must not have the same name as a VDOM.
A VLAN or VDOM must not have the same name as a Zone.
 
Try to make each firewall object name as unique as possible so that it cannot be confused with another object.