As a Russian speaker, I would say that I feel our swear words become truly offensive when they are explicitly targeted at a person. "Dura" is also not considered to be an expletive, and I have not heard it being used in its original meaning after I finished 5th grade. Pronunciation in Russian is also different, word sounds like "doo-ra".
FWIW the same word "dura" may also be used as a slang word for a large and unwieldy inanimate object.
Sometimes the word "dura" has the meaning of "something big and an intricate nature," i.e., just a synonym for "stuff." For example, "положи эту дуру в шкаф" (put this stuff in the closet)
FWIW the same word "dura" may also be used as a slang word for a large and unwieldy inanimate object.