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There isn't much you can do with a CSV file because it's in TXT format. However, one of the things you can do is to load it into an Excel spreadsheet. You can either look for the file and Open With Excel or open Excel, search for the file and open it. If you follow this way the File Open Dialog will show only Excel files by default so that your CSV file isn't displayed for selection. Change the file type selector at the right bottom (just above the Open button) to "All files".
Once your data are in an Excel spreadsheet you have all the capabilities of Excel to manipulate them, including filters to remove duplicates and Find & Replace to remove prefixes. In fact, the problem turns into one you haven't described but it's managable.