1. Have cdb installed.
2. Build Qt 4.8.6 with MinGW 4.9.1.
   I used the MinGW from the Qt 5.4.1 package.
3. Build a project on this Qt.
   I used a default Qt Widgets Application.
4. Have the used MinGW in your PATH variable.
5. Start Creator with empty settings.
6. Open the pro-file of the project from step 2.
   Make sure that there is no pro.user-file. Creator will autodetect the Qt version and create two kits with it:
   
7. Go to "Tools" -> "Options..." -> "Build & Run" -> "Kits" and compare the two kits.
   They are nearly the same. Differences are that the one has gdb set as debugger, whereas the other has cdb and its mkspec is explicitly set to "win32-g++-4.6".

In this case, I think Creator should create only one kit which uses gdb.