emacs client

When one instance of emacs is already running and you want to open files from command line into the same instance, use emacs client.

First, start the emacs server. Add the following line in your .emacs file:
(server-start)

Start emacs, and use emacs client from the command line to open the files in the already open emacs instance:
emacsclient -n filename

It helps to create an alias:
alias cemacs='emacsclient -n'
alias emacs='emacs &'