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hi im not sure which addon I added that give me this error but is there anyway I can slove this problem?

Fatal error: Cannot redeclare user_exists() (previously declared in /home/xiaoxue/domains/test.com/public_html/forum/inc/functions_user.php:9) in /home/xiaoxue/domains/test.com/public_html/forum/inc/functions_user.php on line 9

thanks for ur reply I take alook