How to prevent duplicate entry in a text file using C++

The only way to check is to read every entry in the text file, and check each and every one of them. Text files on their own don't have any organisation - not even lines - so you can't add "extra info" like a hash value to make the process quicker or easier unless you add it to a separate file and maintain that because separate app instances even on the same machine do not share memory.

I had to do something like this just today. What differed was I was comparing two directories and listing the files that one directory was missing but it was essentially exactly what you need to do. I made a list of all the files in the "base" directory and then for every file in the other directory I looked for it in the list and if it wasn't there I displayed it. In my case, that list was actually a vector of strings. I had about two hundred files so I didn't both with a map or binary search and it was plenty fast enough, even in debug mode. I would recommend a similar tactic for you. You can save a vector of your input data items and then search for each new item in it.