With existing facilities in Word
When you paste text in, do you include the URL of the source? If so, before pasting in a block of text from a web page, you could search for its URL, or part of its URL, in your existing document.
Alternatively, you could select a relatively large block of text from the incoming document, e.g. a whole paragraph, and paste it into Word's Find dialog, to see if that's present in your existing document. (Make sure this search ignores formatting)
If none of the above work for you, then I think you are going to need some kind of quite sophisticated text analysis, I suspect. The search term that comes to mind is plagiarism!
(Please note: I am not suggesting that plagiarism is what you are doing - just that it's a reason for people to implement the kind of functionality you are looking for)
So, I think your best bet is going to be to find some kind of tool that detects plagiarism, and to see if you find a way to get it to do what you want.
Possibly useful links:
- WriteCheck - Developed by the creators of Turnitin, WriteCheck helps students check for plagiarism and correct grammar, style and spelling errors with user-friendly reports and helpful resources to improve writing.
- Grammarly - Grammarly is an automated proofreader and your personal grammar coach. Check your writing for grammar, punctuation, style and much more.
I'm going to stop here. There are masses and masses of alternatives on Google, including more Word add-ins.
I'd suggest you have a look at a few, and see if any of them work, or if you can do a more refined search, with more knowledge of what you are trying to achieve, and how flexible you can be in your approach.
(If you find something that works for you, it would be great if you could add a note in this question, for future reference.)