I have my own views on plagiarism and they are not universally shared.
I believe that true plagiarism needs to be an intentional act. I read something and decide that I want to present it as my own original thoughts. That is pretty hard to prove. But it must be avoided if scientific principles are to be followed. Among other things, it breaks the chain of context provided by the original, which can lead to errors later. That is independent, however, of whether sanctions are warranted.
This follows a general rule that ethical transgressions require intent.
One reason that it can be hard to prove is that it is about ideas, not words. It is possible to plagiarize using exactly none of the original words. The intention can be invisible. So, in particular, paraphrasing an original doesn't make plagiarism go away.
But there are also two kinds of "apparent" plagiarism. They are problematic, but don't involve the same sort of intention. And they may be sanctionable or not.
The first is not knowing the rules at all and so not knowing that you have broken them, even if you do something so bold as to copy-paste actually expression (which can also fall afoul of copyrights, but that is a different issue). In reality, if this is the case, someone has failed in teaching you the rules. It is a "we don't know what we don't know" sort of situation. Some people just grok the rules, but others assume that everyone groks them and so this sort of unintentional, obvious, "apparent" plagiarism can be easy to charge and hard to avoid.
And note that copying words can easily make this sort of apparent plagiarism look like a blatant, intentional, act, when, indeed it was, but the "intention" wasn't actually to break an unknown rule, just to copy words blindly.
This sort of thing is what plagiarism checkers look for - copied words, never mind the ideas.
As an aside, I've had student not know some things that seem blindingly obvious to others and that should have been taught. In this case the student's misunderstanding was also reinforced by other factors. But at base, it was bad teaching that led him to error. He hated me for a while for correcting his issues, thinking I was lying to him. And it took me a while to understand that the underlying problem wasn't his.
The other sort of "apparent" plagiarism is just sloppy research practice. "Yes, I know the rules and intend to follow them, but messed up in my writing (focused on other things) and missed the necessary citation. Oh, expletive, I messed up." Sloppy research is, in my view a somewhat (not a lot) lesser offense than deliberate plagiarism. It leads to other issues as well as this one, of course.
I don't know, of course, what you did, or what is being charged. Whether it is deemed serious enough to cancel your degree or not is up to the committee. I expect, however, that at a minimum you might be asked to re-write the thesis taking better care to avoid sloppiness and to follow the rules.
When some publication appears to contain plagiarized material, even if that was unintentional, it needs to be corrected. Sometimes removal is required, sometimes editing might suffice. But, even with "benign" cases, it is important that the public record itself be corrected. But, as I said earlier, it is vital that the "chain of context" be maintained in scholarly writing and even unintentional and "apparent" plagiarism requires correction.
However, and this is why I write this post, I think it is incumbent on any research advisor or supervisor to assure that their students understand the rules. It is simple enough to promulgate them, but students don't coming into a degree program knowing everything that others think they should know. Make a specific point of teaching the rules of the game before you start to play.
There is also another possibility that doesn't involve plagiarism of any kind, but might be misinterpreted as such. That is, the author had no knowledge of the earlier work at all, and reproduced it independently and innocently. That is pretty incredibly unlikely in case the words match, of course. It is also possible that it is a case of sloppy scholarship. "Ah, I shoulda, coulda found that if I'd looked harder."
Independent simultaneous work is pretty common, since the state of the art in a field advances on a front that is known to many. So many might be asking the same questions simultaneously.
Some commenters have suggested that I make a recommendation about what you should do in this situation. It is hard to be definite, since we don't know what is actually being charged or whether it meets the definition of true plagiarism or not.
Once you know more you have some options.
If you did (intentionally) plagiarize you can beg forgiveness, and (hopefully) honestly promise to do better. But the judgement may be harsh if the evidence is strong.
If you (honestly) didn't know the rules you can state that, though whether it would be accepted or not is up to the committee of judges. In that case, they may share blame, but are unlikely (sadly) to acknowledge that.
If you were sloppy in your research or writing, then you can admit to that and it may (or not) gain you a bit. Sloppy research isn't terribly rare, sadly.
In any case, you need to correct the thesis, though that may be moot if the committee rejects any appeal.
You may just have to accept the consequences. Sorry. But you have little control here.
