consider

javadoc test.java test.java


Find the output below:
----------------------

jtools-sqe:/home/vr128054/vertest/work 448 % javadoc test.java test.java
Loading source file test.java...
Loading source file test.java...
Constructing Javadoc information...
test.java:1: duplicate class: test
public class test
       ^
test.java:22: duplicate class: second
class second extends test
^
java.lang.NullPointerException
        at com.sun.tools.javadoc.DocEnv.shouldDocument(DocEnv.java:191)
        at com.sun.tools.javadoc.RootDocImpl.setClasses(RootDocImpl.java:81)
        at com.sun.tools.javadoc.RootDocImpl.<init>(RootDocImpl.java:67)
        at com.sun.tools.javadoc.JavadocTool.getRootDocImpl(JavadocTool.java:205)
        at com.sun.tools.javadoc.Start.parseAndExecute(Start.java:284)
        at com.sun.tools.javadoc.Start.begin(Start.java:114)
        at com.sun.tools.javadoc.Main.execute(Main.java:44)
        at com.sun.tools.javadoc.Main.main(Main.java:34)
javadoc: fatal exception
1 error
2 warnings



While it is valid to report an error, attempting to proceed requires a graceful exit. The command-line parser could perhaps report a warning, then delete the duplicate file name occurrance and give it to the tool for normal processing.