What is the expiation for swearing not to commit Zina, then committing it, and is it permissible to swear again not to commit it?

You must supplicate frequently and sincerely desire repentance, for supplication is the weapon of the believer by which he achieves his goal. If the conditions for supplication and the reasons for its acceptance are met, and the desire for repentance is sincere, then Allah Almighty will be truthful and grant it. A woman must be firm with herself and with this man. A woman who desires to preserve her religion and honor cannot enable a man to communicate with her, let alone be in seclusion with her, and then claim that he forced her to commit indecency. Attention must be paid to the gravity of adultery and its dire consequences in this world and the hereafter, and one must hasten to sincere repentance and fulfill its conditions. It is important to strive for the companionship of righteous women and everything that aids in steadfastness and perseverance, and there is no harm in seeking assistance through an oath to prevent oneself from falling into sin when necessary. Multiple oaths for the same thing necessitate a single expiation upon breaking them, provided no breach occurred before. It is safer for the marriage to take place after repentance.