Is there a term for a words that are written as antonyms, but function as synonyms?

Flammable and inflammable ?

Ravel and unravel.

Unpealing an union.

You are actually pealing it.

@LuckyGuy Thanks for this example. I learned about this word when I was in High School in a radio broadcasting course. It is pretty damn clear what Flammable means—and Inflammable means the same thing—but some people confuse it with meaning “not-flammable” so the word is seldom if ever used. Getting this one wrong could lead to disastrous consequences.

^^^^ that rings a bell of unity

https://en.wiktionary.org/wiki/Appendix:English_contranyms

Great question! :)

Here are a few:

caretaker and caregiver mean the same thing.
inhabitable and habitable (the antonym of both of these is uninhabitable)
deprivation and privation

An example from slang:

to be up for something is the same as being down for it

I don’t know of a specific term for this phenomenon, though.