How do I show the equation formula again instead of its number of ref?

This might be a solution, but I don't see how it would help the reader.

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newcommand{\repeatable}[2]{%
  \label{#1}\global\@namedef{repeatable@#1}{#2}#2
}
\newcommand{\eqrepeat}[1]{%
  \@ifundefined{repeatable@#1}{NOT FOUND}{$\@nameuse{repeatable@#1}$}%
  ~\eqref{#1}}
\makeatother

\begin{document}

\begin{equation}
\repeatable{eq:good}{f(x)=x}
\end{equation}

This is seen in \eqrepeat{eq:good}.

\end{document}

However it seems from your comments that you want something more complicated; please be more specific in your question.

I just found this question that was asked earlier: Is it possible to re-insert a LaTeX equation by label?

It cites "Equation Reuse" from LaTeX-Community Forum as a solution.

Namely:

\documentclass[12pt,a4paper]{article}
\usepackage{amsmath}

\begin{document}

%defining the box
\newsavebox\myeq
%filling the box
\savebox\myeq{\vbox{\begin{align*}
  a &= b\\
  &= c\\
  &= d\\
  &= e\\
  &= f
\end{align*}}}

%using the box
\usebox\myeq

%using the box
\usebox\myeq

%using the box
\usebox\myeq

\end{document}

Hope this helps!