What happens if you install what is already installed?

Question

linux

Good day!
Ubuntu VPS.
Yesterday I had a hot day, due to inexperience I dropped my site several times, several times I wanted to reinstall the system from scratch, but still coped with all the problems without reinstalling, everything works! But at the time when the site was lying, I was in a fever to find a way out and tried to install what was already installed. Here, for example, I used the commands several times:

apt-get install php7.2
apt install php7.2-mysql

Tell me, what is fraught with the installation of installed things?
Thanks to!

Answer 1

Installed things are usually not re-installed. But if the installation fact is not checked (or ignored), then everything depends on the installer script.
Installer:
(if not disabled) checks dependencies and installs what is not
(always) decomposes package files in the places where they are assigned to lie
(if provided) backs up old configs
(if provided) installs new configs, while old ones may be overwritten
( always) updates the information in the database with data about installed programs

Answer 2

than installation of the established things is fraught?

Decrease in storage capacity. Much less often - overwriting configs.