How can I filter out unique results from grep output?

You can achieve this with the sort and uniq utilities.

example:

[john@awesome ~]$ echo -e "test\ntest\ntest\nanother test\ntest"
test
test
test
another test
test
[john@awesome ~]$ echo -e "test\ntest\ntest\nanother test\ntest" | sort | uniq
another test
test

depending on the data you may want to utilize some of the switches as well.

You can use:

grep -rohP "(mySearchString)" . | sort -u

-r: recursive

-o: only print matching part of the text

-h: don't print filenames

-P: Perl style regex (you may use -E instead depending on your case)

sort -u is better than sort | uniq, as @Chris Johnsen pointed out.