Copy string to clipboard and display into output window simultaeneously

To copy the output to the clipboard also outputting to the current terminal you could simply use tee, which will output its stdin to any file passed to it as an argument and to stdout; you can use a process substitution running xclip to "fake" a regular file and output to it instead of to a regular file:

echo -n $GEDIT_CURRENT_DOCUMENT_URI | tee >(xclip -selection clipboard)

Another option is to output to one of the free "default" pseudo-terminals (tty1 to tty6), which have a correspodent device file in "/dev" ("/dev/tty1" to "/dev/tty6"):

echo -n $GEDIT_CURRENT_DOCUMENT_URI | tee >(xclip -selection clipboard) >/dev/tty1

Yet another option is to output to another "listening" pseudo-terminal using a named pipe; this requires a setup to set the "listening" terminal:

First open the "listening" terminal and run this script (for the sake of the example I'll assume that the script is running in ~/tmp):

#!/bin/bash
mkfifo fifo # creates a named pipe named "fifo" in the current working directory
trap 'rm fifo; exit 0' 1 2 3 13 15 # traps SIGHUP, SIGINT, SIGQUIT, SIGPIPE and SIGTERM; removes "fifo" and exits upon the reception of each of them
while [ 1 ]; do
    cat fifo # outputs the content of "fifo"
done

This will create a named pipe named "fifo" in the current working directory and will continuously output its content until the script execution is halted;

Then using Gedit's External Tools run this modified version of the second command, which instead of redirecting the output to "/dev/tty1" redirects it to the named pipe:

echo -n $GEDIT_CURRENT_DOCUMENT_URI | tee >(xclip -selection clipboard) >>~/tmp/fifo

Sample output using two gnome-terminal instances:

Running the script on the right terminal

Running echo -n $GEDIT_CURRENT_DOCUMENT_URI | tee >(xclip -selection clipboard) >>~/tmp/fifo on the left terminal

Hitting CTRL+SHIFT+V

More informations on named pipes