a function has no repeat on its x-coordinates in a points set, now, check this one for any x-repeats,
[tex]\bf \begin{array}{ccll}
x&y\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
-4&0\\\\
\stackrel{rep eat}{-1}&-3\\\\
-3&5\\\\
\stackrel{rep eat}{-1}&1
\end{array}[/tex]