There are no mods in phpBB 3.2, it uses extensions. If I were to guess I’d say it was ABBCode or one of its derivatives. You should ask extension related questions in the extensions forums.
PS/ You’ve probably added it twice one inline and the other not.
"The more connected we get the more alone we become" - Kyle Broflovski
There are no ‘threads’ in phpBB, they are topics.