There are no issues of integrity, morals, ethics, whatever... because it is your work. Furthermore, it is the same assignment from the same source, not a different assignment from a different source. The concept of self-plagiarizing is heavily debated to begin with, but all those arguments tend to be assuming you are using the same piece of work for different purposes without citing that you have done so. (Side note: this is assuming the work was all yours to begin with).

You can even look in the student code here. In the definitions of plagiarism, there is no concept of self plagiarism.

(1) Allowing others to conduct research or prepare any work for a student without prior authorization from the Instructor, including using the services of commercial term paper companies.

(2) Submitting substantial portions of the same academic work for credit more than once or by more than one student without authorization from the Instructors to whom the work is being submitted.

(3) Working with another person without authorization to satisfy an individual assignment. (b) Plagiarism. No student shall represent the words, work, or ideas of another as his or her own in any academic endeavor. A violation of this section includes but is not limited to: (1) Copying: Submitting the work of another as one’s own.

And so on...

That being said, you have to retake the class. Perhaps you should consider paying attention more to assignments rather than less. Just because you took a class before does not exempt you from getting the same or a worse grade. What is the point of even retaking a class if you don't want to relearn the material?