How can we do and mean Teshuvah (lit. “Returning”, the process of Repentence) when knowingly or unknowingly we are going to commit the same sin again?
This is a great question! When a person goes to court would it be fair if the judge passed judgment on what he thought you might do tomorrow or what you have done? Better than that since G-d is G-d He judges us not only on what we have done but rather on who we are right now.
Of course, real Teshuvah means that you never do it again. That is up to your personal free will. If you truly never do it again intentionally then your sins turn into Mitzvos and your Teshuvah works. If not, then it was not sincere Teshuvah to begin with. Either way it not a paradox really because it all depends on the sincerity of the person.
Finally, if a person does something unknowingly then they are not held liable for it at the same level as if they did it knowingly. The bottom line is that Teshuvah, according to the RamBam (Maimonides), means that a person has to admit to themselves, to G-d, and to the person the sin involves (if there is another person) and then commit to never do it again. Some sins need the power of Yom Kippur Day to fully repent and some need more than that (really serious ones). Just know that real Teshuvah can change you forever. There is no such thing as false Teshuvah because if it was not sincere then you never did it.
Rabbi Gershon Litt