Yes, flammable and inflammable mean
the same thing: "easily ignited and capable of burning rapidly,"
according to The American Heritage Dictionary of the English Language
(Houghton Mifflin, 1996).
This dictionary also gives this useful note on p.690:
Historically, flammable and inflammable mean
the same thing. However, the presence of the prefix in- has
misled many people into assuming that inflammable means "not
flammable" or "noncombustible." …it is therefore
advisable to use only flammable in contexts imparting
warnings or on product labels, where a misinterpretation might have
more serious consequences for the reader than an etymological mistake