You're checking out all branches on a case-insensitive filesystem (such as MacOS HFS+), and both Zend and ZEND are branch names. Check out a smaller subset (like only one branch, or only the trunk), or move to a case-sensitive filesystem (you can create a disk image on OS X) if you really want the 30GB full checkout of php-src with all its tags and branches for some reason.
Windows users generally won't have the option to create a case-sensitive filesystem. Can't we just rename one of those two branches to something more descriptive?
S