Attached the standalone sample with sample annotation processor and the source annot02005.java to demonstrate the problem.

The source works well with JDKb20-b03 but throws AssertionError with JDKb20-b04.

with JDK20-b04:
====
javac.exe --enable-preview --release 20 --enable-preview -processor TestProcTypeAlreadyExistsWarning annot02005.java
Inside TestProcTypeAlreadyExistsWarning::process com.sun.tools.javac.model.JavacElements@add0edd
SUCCESS
An exception has occurred in the compiler (20-ea). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
java.lang.AssertionError
        at jdk.compiler/com.sun.tools.javac.util.Assert.error(Assert.java:155)
        at jdk.compiler/com.sun.tools.javac.util.Assert.check(Assert.java:46)
        at jdk.compiler/com.sun.tools.javac.comp.Annotate.lambda$annotateLater$0(Annotate.java:253)

.....

With JDKb20-b03
============
javac.exe --enable-preview --release 20 --enable-preview -processor TestProcTypeAlreadyExistsWarning annot02005.java
Inside TestProcTypeAlreadyExistsWarning::process com.sun.tools.javac.model.JavacElements@1460a8c0
SUCCESS
Inside TestProcTypeAlreadyExistsWarning::process com.sun.tools.javac.model.JavacElements@1460a8c0
SUCCESS