I have a question on StackOverflow that was answered and I thought I understood everything, but now I have another question regarding the same exact topic.
Am I allowed to re-comment on the question?
You can comment on the original question.
However, you'll get better results if you open a new question. As part of that, link back to the original, and make sure to emphasize what makes the new question unique, or the new question will be rightfully closed as a duplicate.
For example, say you originally asked how to accomplish some task, and got back a few recommendations. You accepted one and moved on. Now you are wondering how to distinguish when to use which recommendation. That's fine as a new question, but make sure you ask it appropriately. Don't re-ask the original question. Make sure your new question focuses on how to distinguish or evaluate between the various options.