Flammable/Inflammable
The International Maritime Dangerous Goods Code, which is the Bible for the transport by sea of hazardous materials, begins the section on flammable materials like this:
"CLASS 3 - FLAMMABLE Liquids
1. PROPERTIES
Definition of flammable* liquids
[footnote] *"Inflammable" has the same meaning as "flammable".

Since the IMDG regulates all oceangoing transport of hazardous goods, and matters of very large import and many millions of dollars hang on its provisions, they take no chances on anyone not knowing about either of the two expresssions which mean the same thing. This is, of course, the way lawyers use language.