This issue can be reproduced with jdk14 and jdk15.
jdk-14.jdk/Contents/Home/bin/jpackage --input ./input --dest output --name Test --type dmg --main-jar hello.jar --main-class Hello --mac-sign --mac-signing-key-user-name jpackage.openjdk.java.net --mac-signing-keychain jpackagerTest.keychain --verbose
will fail with error below:
/var/folders/wv/v95v04852ssgxhyxm38xp9lh0000gn/T/jdk.incubator.jpackage15520659922256584206/images/image-4513431408695035725/Test.app/Contents/runtime: is already signed
java.io.IOException: Command [codesign, -s, Developer ID Application: jpackage.openjdk.java.net, --prefix, Hello., -vvvv, --keychain, jpackagerTest.keychain, /var/folders/wv/v95v04852ssgxhyxm38xp9lh0000gn/T/jdk.incubator.jpackage15520659922256584206/images/image-4513431408695035725/Test.app/Contents/runtime] exited with 1 code