Tl;dr: One should not use a label already used in some other post in the same thread because when a thread loads, all MathJax codes are processed at the same time and so MathJax throws an error if there are two equations with the same label (If such an issue has not been discussed on Meta.Math.SE, it may not be a bad idea to start a new thread talking about it because it can be a widespread one among users).

Solution: You can use the "tag" code alone, as explained in the above answer. However, since you are a high rep user and your edit would be implemented without being reviewed, I suggest that you change the label in the OP to another one, for example, "*", and so change them in the "\ref"'s in it.

It is worth noting that there are some MathJax-rendering bugs having nothing to do with the codes involved in a post. Some browsers on some devices render them perfectly and some cannot. For example, see the following post: