Add explicit comments for duplicated lines

Explain the reason to avoid further deletion.

Install/fish-install

@@ -3,6 +3,6 @@
 # Install fish
 
 sudo apt install fish
-sudo chsh -s /usr/bin/fish
-chsh -s /usr/bin/fish
+sudo chsh -s /usr/bin/fish # change shell for superuser
+chsh -s /usr/bin/fish # change my shell