This error is related to cross-referencing. It appears when you use the same label multiple times for different objects, causing LaTeX to get confused when \ref{...} is used.

Common Causes

Same label used multiple times:

The most basic example of such an error is simply giving the same label to different objects as shown below

\section{Section 1}
\label{algebra}

\includegraphics{image.jpg}
\label{algebra}

Here, both the image (image.jpg) and the section (Section 1) have been labelled as algebra, causing LaTeX to get confused as to which one to reference when \ref{algebra} is used. This will lead to an error as shown below

To resolve this, rename one of the labels to something else.