To reproduce, checkout the langtools repository so that any of its parent directories ends with "com" (e.g. "/tmp/test.com"). Building langtools then fails with:
---
$ (cd make/; ant clean build-all-tools)
...
    [javac] Compiling 294 source files to /tmp/test.com/langtools/build/bootstrap/jdk.compiler/classes
    [javac] /tmp/test.com/langtools/build/bootstrap/jdk.compiler/gensrc/com/sun/tools/javac/resources/CompilerProperties.java:22: error: duplicate class: com.langtools.src.jdk.compiler.share.classes.com.sun.tools.javac.resources.CompilerProperties
    [javac] public class CompilerProperties {
    [javac] ^
    [javac] /tmp/test.com/langtools/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/DeferredAttr.java:31: error: cannot access CompilerProperties
    [javac] import com.sun.tools.javac.resources.CompilerProperties;
    [javac] ^
    [javac] bad source file: /tmp/test.com/langtools/build/bootstrap/jdk.compiler/gensrc/com/sun/tools/javac/resources/CompilerProperties.java
    [javac] file does not contain class com.sun.tools.javac.resources.CompilerProperties
    [javac] Please remove or make sure it appears in the correct subdirectory of the sourcepath.
    [javac] /tmp/test.com/langtools/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Types.java:55: error: cannot find symbol
    [javac] import static com.sun.tools.javac.jvm.ClassFile.externalize;
    [javac] ^
    [javac] symbol: static externalize
    [javac] location: class
---