[Tex/LaTex] Hyperref and nonumber/notag in equations:

alignhyperrefnumbering

When compiling my text document, I get the following warning:

[5] [6] [7pdfTeX warning (ext4): destination with the same identifier (name{equ
ation.2.0.10}) has been already used, duplicate ignored

\AtBegShi@Output ...ipout \box \AtBeginShipoutBox 
                                              \fi \fi 
l.210     \end{equation}

A quick google search reveals that this is a problem hyperref has with suppressed equation numbers: It suggests using 

\begin{align*} ... \end{align*}

instead of 

\begin{align} ... \notag \end{align}

Which has the same visual effect of not displaying an equation number. So far, so good.
But what if I want to have an equation number for the last line of my align-environment, and only suppress the numbering of all the preceding lines?

I'm also confused about this warning occurring only once, although I use \notag all the time.

Here is a minimal example:

\documentclass{report}

\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[T1]{fontenc}

\usepackage[pdftex, colorlinks=true,bookmarksnumbered=true]{hyperref}
\usepackage{amsmath}

\usepackage[hyperref, amsmath, thmmarks]{ntheorem}

\begin{document}
  \begin{equation}
    \begin{split}
        (a+b)^2 = \\
        a^2 + 2ab + b^2
    \end{split}
  \end{equation}
\end{document}

The offending package is ntheorem and the offending command is the split environment if I remove either of the, I get no warning any more.

Best Answer

At least for your minimal example, it's enough to replace equation environment with gather. As the hyperref readme says: \let\equation\gather \let\endequation\endgather. Also you should load hyperref after amsmath (usually hyperref should be the last package loaded, but ntheorem is special and should come after).

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