What is the expiation or ruling for someone who swore not to commit a forbidden act, saying: "If I commit this act, then upon me is the expiation for an oath by fasting three days," and then added: "This expiation is upon me every time I commit this forbidden act," and he has now committed this act twice?

What you uttered is a vow of contention (nathr al-lajaj), and its maker has the choice between fulfilling the vow or offering an expiation for an oath (kaffarat yamin). Since what you swore to abandon is forbidden, you must abandon it. If you fall into it, and you had vowed an expiation for an oath, then you must offer it. And since you uttered words implying the repetition of the expiation, it becomes binding upon you for every time you commit the aforementioned sin.