Two equations with same label

Question

I have to translate a dissertation to a researcher. There is a problem I couldn't solve. Namely, there is an introduction chapter where the author has named an equation as (A). Then in the chapter one there is again an equation named as (A). Those equations are not the same in sense that first one has parameter z but another has parameter e^z. So how can I name two different equations with the same label? For example

e^{ix}+1=0 (A)

and

$e^{iy}=-1$ (A)

Answer 1

If you are using the amsmath package, you can specify the displayed label using the \tag macro:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
    \begin{equation}\tag{A}
        e^{ix}+1=0
    \end{equation}
    \begin{equation}\tag{A}
        e^{iy}+1=0
    \end{equation}
\end{document}

Answer 2

Let's look at a possible scenario.

The text has an unnumbered first chapter, where equations are identified by letters, while the body of the book has equations numbered like (chapter.equation).

\documentclass[oneside]{book}
\usepackage{amsmath}

% this and 'oneside' is just for making small pictures
\usepackage[a6paper]{geometry}

\numberwithin{equation}{chapter}

\begin{document}

\frontmatter
\renewcommand\theequation{\Alph{equation}}

\chapter{Introduction}

Some text
\begin{equation}\label{eq:Euler}
e^{ix}+1=0
\end{equation}
some text

\mainmatter
\renewcommand\theequation{\thechapter.\arabic{equation}}

\chapter{Title}

Some text followed by an equation
\begin{equation}\label{eq:easy}
1+1=2
\end{equation}
and here we use an equivalent formulation of an equation in the introduction
\begin{equation}\tag{\ref{eq:Euler}}
e^{iy}=-1
\end{equation}
Some other text

\end{document}

Using \ref in the recalled equation allows to make this independent of the actual number used in the introduction.

The author might have not used \renewcommand\theequation and have assigned “A” manually with \tag; but the result would be the same

% in the introduction
\begin{equation}\label{eq:Euler}\tag{A}
e^{ix}+1=0
\end{equation}

% in the body
\begin{equation}\tag{\ref{eq:Euler}}
e^{iy}=-1
\end{equation}

Answer 3

If you want to reset the number of the equations conter you could use:

\setcounter{equation}{0}

right before the second equation (A).

If you want to use the equation number A just for that equation and keep with normal numbering you could use:

\begin{equation}\tag{A}
  e^{iy}=-1
\end{equation}