If I remember correctly this happened some time ago to me to (with Google Toolbar or something).
Why did this happen? That's easy: Programs can store their uninstall-information in two parts of the registry: HKEY_LOCAL_MACHINE and HKEY_CURRENT_USER - ZSoft Uninstaller (ZU) reads both. If a program decides to add its uninstall-info both places it will be added to the list in ZU twice.