FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 190588
This article explains how since FortiOS 5.6, it is possible to create VIP with same External IP and Mapped IP.
However, until firmware 6.0 creating VIP with same External IP and Mapped IP will throw an error in both CLI and GUI.

CLI Error:
Static NAT's extip should be different from mappedip.
object check operator error, -8, discard the setting
Command fail. Return code -8

GUI Error:

Change the VIP Type from static-nat to load-balance while creating VIP.
Disable the arp-reply, which is enabled by default.

Via CLI:

#config firewall vip
     edit "SSH_redirect"
          set extip
          set extintf "any"
          set portforward enable
          set mappedip ""
          set extport 666
          set mappedport 22
          set arp-reply disable  
          set type load-balance  
Via GUI:

Create a new VIP, because if the change is processed on an existent VIP's Type from static-nat to load-balance an error will occur.

The difference between static-nat and load-balance is that load-balance is used to balance the traffic between different Servers behind a FortiGate.
But if only one-to-one DNAT mapping is used, it will work the same way.