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I have an equation like:

\begin{equation} \label{whatever}
x+y=1
\end{equation}

I want to write it again, keeping the same number. I know I could do the following:

\begin{equation} 
x+y=1 \tag{\ref{whatever}}
\end{equation}

However, I would like to get the same outcome without rewriting the full equation. The reason is that I may change the first equation, and I would like the change to be automatically applied to all the cases in which the equation is repeated. I.e. I'd like to have something like

\begin{equation} 
\rewrite{whatever} \tag{\ref{whatever}}
\end{equation}

Is it possible?