How does the Fortigate firewall deal with an XFF header inserted

Hi; I would like to have Fortigate insert an X-Forwarded-For header by using the "Preserve Client IP" option available with a load balancing virtual server. My question is: If there is an upstream device that inserted the XFF header before the Fortigate, does the fortigate add a second value to the first. So:

X-Forwarded-For: value1 inserted by upstream device, value2 inserted by Fortigate

Or does it add a second occurance of the XFF header, like:

X-Forwarded-For: value1 by upstream device

X-Forwarded-For: value2 by Fortigate Kindly Wasfi