Fortigate integration with Active Directory Problem

Good Morning 

I hope You are doing well.

I have a problem in integration  I installing of FSSO and DC Agent on AD and configure LDAP Server , Fabric connector and creation policy to access internet 

the user is working fine but 15 mins the Captive portal page appears .

I logged on the  AD ,check the login user on FSSO of the ip address and I found another user logged in with my   IP address  so i have to logged out and logged in again what is the problem exactly