You can redirect error output from the workstation screen to a file. For example:

(sort -u filea >dev/tty) >& outerr

sorts filea, checking for unique output records. Any messages regarding duplicate records are redirected to a file named outerr.

And if you do not care about seeing the error output, you can just redirect it to /dev/null, also known as the bit bucket. This is equivalent to discarding the error messages.

(sort -u filea >/dev/tty) >& /dev/null