The common source of this issue is MathJax and the copy system of your browser. Formulas written as MathJax tags get parsed by MathJax and then replaced by complicated mixture of HTML tags and style sheet commands (CSS). When you copy text containing such "styled" formulas, the browser copies the selected text in different formats (text, unicode text, html tags, and some more). But the clipboard sometimes does not understand this exotic mixture and constructs the best it can, which by unknown reasons sometimes doubles the content, at least in the formats that our editor understands. There is no workaround or code I can provide to prevent this behavior.
However, there are two possible ways I know to securely copy and paste formulas on PhysicsOverflow:
- Right-click the formula, select "Show Math As" and then "TeX Commands". Select the text displayed and copy it. Then paste it into the editor and add dollar signs, as usual. I am not sure if this method works in all browsers.
- Edit the post containing the formula using our editor, copy the MathJax tags there and paste them into the editor containing your new post. Maybe you have to open two tabs on your browser for convenience.