Thanks for your very pertinent and fast answer.
But when you say :
"But no matter the sad practices, under no circumstance does a Ubuntu package install itself twice on your system. As long as you remain true to the official repositories, and make sure never to use two different PPAs for the same software, you don't get duplicate installations of software."
Then how come right after installing Ubuntu MATE yesterday, I installed Tor Browser from the Software Center with no system update beforehand, then I did the big system update, and there, in the Software Center, I was proposed to install Tor in the Internet category while I had it already installed and still appearing in the menu and all working fine ?
Thanks for teaching me these details that make the difference.