1. Open a CMake-based project.
   I used a Qt Widgets Application from Creator's own wizard.
2. In Project mode, add a new build configuration, e.g. of type "Release".
   The build configuration "Release2" will be created. It uses the build directory "(...)\build-(projectname)-Desktop-Release".
3. Add another build configuration of the same type.
   The build configuration "Release3" will be created. It uses exactly the same build directory.

I expect that reusing the same directory again and again for different configuration will lead to trouble. Please use separate directories as Creator does for qmake-based projects.