# Aligning equation error

I used the environment \begin{align}...\end{align} to align some equations, it seems everything is correct (MWE), but I got the following error:

Package amsmath Error: Multiple \label's: label 'eq:Equat_2' will be lost

What is the problem?

MWE:

\documentclass[a4paper,french,11pt,oneside]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[french]{babel}
%----------------------------------------------------------------
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{bm}
\usepackage{mathtools}
%-----------------------------------------
%\usepackage{a4wide}
\usepackage{amsfonts}
\usepackage{amstext}
\usepackage{amsthm}

\begin{document}

\begin{align}
\mathbf{X}_{i,k|k-1}
& \!\begin{aligned}[t] &=
\hat{\mathbf{x}}_{k|k-1} +\mathbf{S}_{k|k-1}\bm{\xi}_i
\end{aligned}\\
\label{eq:Equat_1}
\mathbf{Y}_{i,k|k-1}
& \!\begin{aligned}[t] &=
\mathbf{h}(\mathbf{X}_{i,k|k-1},\bm{\theta})\\
\end{aligned}\\
\label{eq:Equat_2}
\hat{\mathbf{y}}_{k|k-1}
& \!\begin{aligned}[t] &=
\frac{1}{m}\sum_{i=1}^m \mathbf{Y}_{i,k|k-1}
\end{aligned}
\label{eq:Equat_3}
\end{align}

\end{document}

• you don't need aligned here at all, but where you do need aligned you don't need to prefix with \! unless you are using a version dated prior to 2016. Jan 4, 2020 at 19:43

The problem is that you have two \labels within the third equation. Here's a commented view of your code:

\begin{align}
\mathbf{X}_{i,k|k-1}
& \!\begin{aligned}[t] &=
\hat{\mathbf{x}}_{k|k-1} +\mathbf{S}_{k|k-1}\bm{\xi}_i
\end{aligned}\\
\label{eq:Equat_1} % <--- \label for SECOND equation
\mathbf{Y}_{i,k|k-1}
& \!\begin{aligned}[t] &=
\mathbf{h}(\mathbf{X}_{i,k|k-1},\bm{\theta})\\
\end{aligned}\\
\label{eq:Equat_2}% <--- \label for THIRD equation
\hat{\mathbf{y}}_{k|k-1}
& \!\begin{aligned}[t] &=
\frac{1}{m}\sum_{i=1}^m \mathbf{Y}_{i,k|k-1}
\end{aligned}
\label{eq:Equat_3}% <--- \label for THIRD equation
\end{align}


Note that a \\ moves the construction to the next equation. That's why \label{eq:Equat_1} is actually a \label for the second equation, not the first.

Here is a far clearer implementation of your alignment; you don't need the nested aligned constructions:

\documentclass{article}

\usepackage{amsmath,bm}

\begin{document}

\begin{align}
\mathbf{X}_{i, k \mid k - 1}
&= \hat{\mathbf{x}}_{k \mid k - 1} + \mathbf{S}_{k \mid k - 1} \bm{\xi}_i
\label{eq:Equat_1} \\
\mathbf{Y}_{i, k \mid k - 1}
&= \mathbf{h}(\mathbf{X}_{i, k \mid k - 1}, \bm{\theta})
\label{eq:Equat_2} \\
\hat{\mathbf{y}}_{k \mid k - 1}
&= \frac{1}{m} \sum_{i = 1}^m \mathbf{Y}_{i, k \mid k - 1}
\label{eq:Equat_3}
\end{align}

\end{document}