rahat rahat is very informal. those kinds of repeated adverbs are usually informal.
I think rahat rahat has a slightly other meaning. As to my knowledge rahat rahat means "very calmly"
Okursun is present tense, the correct answer should actually be "you will read this book easily" I reported that correction to Duo.
In the Turkish sentence yes, "will" (not "would") isn't a separate explicit word present in the sentence, it is implied as you said. But when translated to English the implication of "will" (not "would") is lost entirely, that is why the correct translation should be "You will read this book easily" for it to fully reflect the meaning of the sentence in Turkish.
If the "correct" answer as it stands is left alone, it can easily be misread/misinterpreted as past tense "You read this book easily".
FYI: The reason it is "will", and not "would" as you said, is because "okursun" is in the present tense; "would" be correct if it had been "okurdun".
Okursun = you read(2nd singular) or you would read Okuyorsun = you are reading
The future prefix is: "acak". Why is it not either "kolaycak" or "Kolayacak ? Is it an exception? I just need to know if there are exceptions in the future prefixes also. Thanks.