Synonyms that are Identical Except for Prefix

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What would you call a word that has an identical word except for the prefix that means the exact same thing?

For example: meliorate and ameliorate both mean the same thing flammable and inflammable both mean the same thing

and so on.

I ask in preparation for the GRE. I was given in my study flashcards the word ameliorate, and then in the practice test I had to know what meliorate means ... and I mistakenly assumed it was the opposite of ameliorate. I would like to know the term for this type of word, so I can find on Google a solid list of such words, in preparation for if the GRE throws this wrench again.