When trying to issue a make clean, the makefile fails

ocarina-main.gpr:41:09: unit "ocarina.configuration" cannot belong to several projects
ocarina-main.gpr:41:09: project "ocarina.main", "/home/julien/wip/ocarina/src/main/ocarina-configuration.adb"
ocarina-main.gpr:41:09: project "ocarina.config", "/home/julien/wip/ocarina/src/config/ocarina-configuration.adb"
gnatclean: "ocarina-main.gpr" processing failed
Makefile:610: recipe for target 'ocarina-main.gpr-clean' failed
make[1]: *** [ocarina-main.gpr-clean] Error 4
make[1]: Leaving directory '/home/julien/wip/ocarina/projects'
Makefile:402: recipe for target 'clean-recursive' failed
make: *** [clean-recursive] Error 1