Oh, this language of ours!

It's always been a source of confusion that the English words "flammable" and "inflammable" mean the same thing.  But these are not the only such pairs of words - you also, for example, have "habitable" and "inhabitable" and  "valuable" and "invaluable".  The confusion arises because the prefix in- most often has a negative connotation as in "inexpensive" (not expensive) but here it is simply an intensifier adding to the meaning of the word - so if there is a difference in meaning it is that "flammable" means "catches fire easily" and inflammable means "catches fire very easily".  The distinction can perhaps be best seen in "valuable" (having a high value) and "invaluable" (beyond price).