Package amsmath Error: Multiple \label's: label 'gleichung' will be lost. Trotz einmaligem Label?

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\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{gather}
  \begin{aligned}
    (a + b)^2 &= (a + b) (a + b) \\
              &= a^2 + 2 a b + b^2 \label{gl:c}
  \end{aligned} \\
  (a + b) (a-b) = a^2 - b^2
\end{gather}
\begin{gather}
  \begin{aligned}[b]
    (a + b)^2 &= (a + b) (a + b) \\
              &= a^2 + 2 a b + b^2 \label{gl:b}
  \end{aligned} \\
  (a + b) (a-b) = a^2 - b^2
\end{gather}
\begin{gather}
  \begin{aligned}[t]
    (a + b)^2 &= (a + b) (a + b) \\
              &= a^2 + 2 a b + b^2 \label{gl:t}
  \end{aligned} \\
  (a + b) (a-b) = a^2 - b^2
\end{gather}
Siehe \eqref{gl:c}
\end{document}
 
 
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