Using \label with amsmath nested environments

Using align inside gather is officially supported (see p. 23 of amsldoc.pdf, the end of section 10.2). However, using \label inside the inner align is buggy: if \notag is used, then the error is raised, if \notag is not used, a label in the second line is duplicated.

A horrible hack is to disable the error generation:

\makeatletter
\def\label@in@display{\gdef\df@label}
\makeatother

This of course will suppress the error also when it's legitimately raised, so use with care.

clearly this is a bug, and will be investigated as such.

but here's another horrible hack that merely gives the nasty warning that

LaTeX Warning: Label `important' multiply defined.

at least you wouldn't have to press enter every time ...

\documentclass[a5paper]{article}
\usepackage{amsmath}
\begin{document}
\begin{gather}
\begin{align}
a &= b + c \tag*{} \\
b - a &= -c
\label{important} \\
c - a &= -b
\end{align}\\
a - (b-a) - (c-a) = 2a
\end{gather}
some text with an xref: \eqref{important}
\end{document}