How to get only the unique results without having to sort data?

$ cat data.txt aaaaaa aaaaaa cccccc aaaaaa aaaaaa bbbbbb $ cat data.txt | uniq aaaaaa cccccc aaaaaa bbbbbb $ cat data.txt | sort | uniq aaaaaa bbbbbb cccccc $ The result that I need is to **display all the lines from the original file removing all the duplicates (not just the consecutive ones), while maintaining the original order of statements in the file**. Here, in this example, the result that I actually was looking for was aaaaaa cccccc bbbbbb How can I perform this generalized uniq operation in general?