If you have multiple files and you want to remove duplicate values from, while creating a single file.
File1 (one.txt):
123
123
234
345
File2 (two.txt):
123
678
567
890
As you can see, the two files contain some duplicates. You can easily remove these using the following command:
sort -u one.txt two.txt
This will output:
123
234
345
567
678
890
You can as easily pipe this into a third file as follows:
sort -u one.txt two.txt > three.txt
This will take the first two files (you can add as many as you like), and pipe the output into a file called three.txt.
Simple!