Why does flammable mean the same as inflammable?

The first word to be used was the word "Inflammable" based on the latin words, meaning "to kindle to burn/fire". in- to kindle flamma - to fire/burn After 19th century, flammable was defined based on the latin word "flammare" which means "to set on fire". So both the words mean the same thing, something that is easy to burn. Now if you want to know about a word which cannot be easily burnt, Inflammable is not the right word. The right word is Non-Flammable. Check out the Oxford dictionary if am wrong.