Is it bad to apply to the same PhD program two years in a row?
You are absolutely allowed to reapply to schools in successive years. However, unless you have had a substantial updated in your profile, such as through improved grades, or additional work or research experience, I would not necessarily expect a better result.

However, it is definitely possible, and if there has been a substantial improvement, the results can be very different. (I've seen firsthand examples of this in students for whom I've written letters of recommendation!)