In this level the password is stored in the file data.txt and is the only unique line of text that occurs only once.
For this we use the command uniq -u. This command filters out all lines that occur more than once in the file.
uniq -u data.txt
The output isn't as expected:
The reason we're not getting a single output is that uniq cannot find duplicate lines if they are not adjacent. To achieve this we can sort the file. We can then pipe the output of sort into uniq -u to find the unique line.
sort data.txt | uniq -u
The output is a single line, revealing the password: EN632PlfYiZbn3PhVK3XOGSlNInNE00t