Is $\Pi’\propto_{poly}\Pi$ for any two NP-complete problems $\Pi,\Pi'$?
Try to combine the following two definitions:

A problem $\Pi$ is NP-hard if $\Pi' \propto_{\mathit{poly}} \Pi$ for every $\Pi'$ in NP.
A problem $\Pi$ is NP-complete if $\Pi$ is NP-hard and $\Pi$ is in NP.