In case of module has two packages with the same name, at least one of them by exports statement from another module, compiler returns warnings, but error is expected due to:
1) Modular Visibility and Accessibility wiki page:
Resolution may also fail if it computes a graph of modules where multiple modules export a package of the same name.
2) API spec of Configuration class, resolve() method:
Resolution and the post-resolution consistency checks may fail for several reasons: Two or more modules in the configuration export the same package to a module that reads both. This includes the case where a module M containing package P reads another module that exports P to M.

MINIMAL TEST CASE

Directory structure:
mod1/module-info.java
mod1/pack/B.java
mod2/module-info.java
mod2/pack/A.java

mod1 module-info:
module mod1 {
    requires mod2;
}

mod2 module-info:
module mod2{
    exports pack;
}

Command:
...bin/javac.exe -modulesourcepath . -d ../out mod1/pack/B.java mod2/pack/A.java

Result:
warning: package exists in another module: mod2
warning: package exists in another module: mod1
2 warnings

Tested with jigsaw b92 on Windows x64.
Same issue on Solaris.