You (or another user) has added this project to the source code control provider. However, when you tried to add files to the project, a different file with the same name was already in the project. You should use the source code control provider's difference command to see what the differences are, and then check in the proper version.
Rather than using the Add Project command, you could also get the entire project to your local drive, then check out the files you need. Using this method, you would get back in synchronization with the project, but would lose any local changes you had made to the project files.