I have a WIP that's on its third series title. After choosing the first title, there was a major movie series with a very similar title, so I changed it, making the title of book one the new series title. Then, some years later, that exact title was also TV series and a movie or something too. So, I came up with a new series title which, so far, no one else has used that I've seen. I also secured the .com domain name so I imagine if there were to be a big movie or TV series wanting that time before I publish, there's a good chance they might contact me about buying the domain name. Maybe. Anyway, I'm keeping the book one title because that title has been used often enough now that I don't think it will be a problem since the series title should serve to differentiate it.