What is the reason for repeatedly committing a specific sin that I swore to abandon, and is what I did permissible, and what are the consequences for me, knowing that I committed it several times after taking the oath?

If someone swears not to do something, then swears not to break that oath, and then does it, they owe two expiations: one for breaking their first oath, and another for breaking their oath not to break the first one. Repeated actions after the first breaking of the oath do not necessitate repeated expiation, because the oath has already been dissolved. One must repent of the sin and guard their oaths.