There isn't a complete solution to this. But clearly this guy hasn't done his homework. If someone has a similar idea then yes you scrap it specially if the concept is similar which in this case is the oak leaf making up the mountain. When people do searches for similar marks, you cant possibly be 100% certain you saw everything but if an idea has been rehashed it is most certainly going to show up in your first few searches. Nobody is saying this is an easy task, some of us spend days scouring to make sure there isn't one, hell clients help you with the search sometimes. To address your other question, if there's a mark that looks similar and is being used by a client, it depends on a lot of things before you decide to pull the plug. If money is spent, then it makes mattters worse. I'd have to leave a long ass post to answer that question really. But to keep it short, lets take tarter's logo, they've been around for yrs but are not exactly prolific outside of farming as far as I know. Get on dribbble, and look for "usa" "america" logos and you'll see a lot of them have rehashed the E as a flag concept. So as a designer I'd withdraw mine if I saw tarter's logo, as a client I'd be pissed if there's something similar and may not withdraw it if we're in different industries, if i've spent money on ads, marketing yada yada, or if tarter doesn't operate in my region. Point being it's not so cut and dry that you tell the client to stop using the mark, it depends on a lot of things. With the OP' that's not the case, atleast from what I can tell the company is non-existent as of now, they're just starting out I guess so its the perfect time to redo the logo before the client starts to invest.