What is the ruling on one who vowed not to do something and then did it, then did it again without specifying the vow each time?

You were obligated to fulfill the vow, unless it involved disobedience. The obligation to fulfill applies specifically to absolute vows. If the vow was not fulfilled, then expiation for breaking an oath is required, based on the Prophet's (peace be upon him) saying: "The expiation for a vow is the expiation for an oath." The expiation is not repeated unless you intended for it to be.