PHP Warning: Module 'json' already loaded in Unknown on line 0

After installing a LAMP server on an old P3 800MHz machine - yes it is so old that it was picked up from my apartment storage room, I typed "php -v" and it says in the first line :

PHP Warning: Module 'json' already loaded in Unknown on line 0

It seems to be the modules json is loaded twice, I looked into the php.ini files and there it is:

extension=json.so

Simply commenting out that line solves the problem and yet leaves me the doubt where is it first loaded besides the time php.ini calls it. I guess there is something with the php5-json package I installed, it may already be included in php5 library and automatically loaded. That is not bad at all since the Debian team will maintain that package and give me the latest one. :D