# Siunitx and breqn causes repeated value and unit error in align

I am experiencing a weird bug when using \SI{}{} inside a align block. The following latex code results in the value and unit being repeated as seen on the picture. But only when written inside a align block.

\documentclass[a4paper, 11pt]{article}
\usepackage{amsmath}
\usepackage{breqn}
\usepackage{siunitx}

\begin{document}
\centering
\begin{align}
f_{n} &= \SI{311.2}{\radian / \second} \nonumber \\
&= \SI{49.5}{\hertz}
\end{align}
\end{document}


Results in:

I have not experienced this kind of behaviour on another machine and even in another document on the same machine.

What could cause this behaviour? And what can i do to stop it?

EDIT 1: Adds a complete example.

In construction of the example i figured out that it is the package breqn which causes the error. But then i would like to ask, why does this happend, and what can i do about it?

Also, I have updated the title of the question to reflect change in question.

• works fine for me. Show a complete example. Jan 7 '19 at 14:54
• All works fine here: can you add \listfiles to your input and then edit the File list from your .log into the question? Jan 7 '19 at 15:23
• I think the general consensus nowadays is the you should not use breqn, the idea is nice, but in reality, its goal is very very hard to achieve and as such it is incompatible with a lot of stuff. Since we are seeing things four times, I'm guessing it is messing with \mathchoice or similar (which basically has to typeset everuthing 4 times before it knows which is is being used) Jan 7 '19 at 15:25
• I can't reproduce the behavior too. You might want to consider using \per instead of a /. I think it works better, but it's your choice. If you want to show a slash instead of negative exponents within the units, you can add the option \usepackage[per-mode=symbol]{siunitx} or \SI[per-mode=symbol]{}{} for a single unit. See the sinunitx package manual for more information. Jan 7 '19 at 15:50

In construction of the example i figured out that it is the package breqn which causes the error.