I have two software centers in Ubuntu, but I am sure there should be one:
And Moreover both have the source "ubuntu focal-main". Can anyone help?
If you install multiple desktop environments then this thing happens.
Gnome supports gnome software centre, but the distro maker changes it a bit and introduces it as another software centre. So if you do sudo apt install gnome-software
then the default gnome software centre is installed along side the one shipped by the manufacturer.
Have you by chance installed some other desktop environment like xubuntu? This is the gnome software center. Ubuntu’s one is a front end made by Canonical. Nothing to worry about here.