How to find and remove duplicate strings of alphanumeric characters from multiple files?

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PowerShell

How to find and remove duplicate strings of alphanumeric characters from multiple files for both alphanumeric strings on consecutive rows as well as if they are not on consecutive rows?
It should be -
1. For complete lines which are case sensitive
2. For multiple lines but I want to do one line at a time so that I can check after each command (to find and remove) is executed
3. Remove all duplicates but keep the original

What I have tried:

Notepad++ but it is removing all the important lines also, for example, the brackets in the CSS section