While a jar file created with jar utility would not have duplicate entries, it is possible to produce a jar file using zip to have duplicate entries.

Duplicated entries in a jar file could lead to override desired content, while JDK-8335912 add option to not override existing files, there should be a way to find out such "malformed" jar file for precaution.

jar have a --validate mode to check jar file consistency, we can also check if there are duplicate entries or inconsistent headers.