diff --git a/plexus-java/src/main/java/org/codehaus/plexus/languages/java/jpms/LocationManager.java b/plexus-java/src/main/java/org/codehaus/plexus/languages/java/jpms/LocationManager.java
index 9a51ae6..06f5683 100644
--- a/plexus-java/src/main/java/org/codehaus/plexus/languages/java/jpms/LocationManager.java
+++ b/plexus-java/src/main/java/org/codehaus/plexus/languages/java/jpms/LocationManager.java
@@ -34,6 +34,7 @@
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Set;
+import java.util.function.Consumer;
 
 import org.codehaus.plexus.languages.java.jpms.JavaModuleDescriptor.JavaProvides;
 
@@ -249,22 +250,47 @@ public String extract(Path path) throws IOException {
                     request.isIncludeStatic());
         }
 
-        // in case of identical module names, first one wins
+        Map<T, JavaModuleDescriptor> definedModules = new LinkedHashMap<>();
+        Map<T, JavaModuleDescriptor> automaticModules = new LinkedHashMap<>();
+
+        pathElements.forEach((k, v) -> {
+            if (v != null && !v.isAutomatic()) {
+                definedModules.put(k, v);
+            } else {
+                automaticModules.put(k, v);
+            }
+        });
+
+        // in case of identical module names, first one wins, others drop onto classpath
+        // if they are automatic modules.
         Set<String> collectedModules = new HashSet<>(requiredNamedModules.size());
 
-        for (Entry<T, JavaModuleDescriptor> entry : pathElements.entrySet()) {
-            if (entry.getValue() != null
-                    && requiredNamedModules.contains(entry.getValue().name())) {
-                if (collectedModules.add(entry.getValue().name())) {
-                    result.getModulepathElements()
-                            .put(
-                                    entry.getKey(),
-                                    moduleNameSources.get(entry.getValue().name()));
+        Consumer<Map<T, JavaModuleDescriptor>> moduleAcceptor = moduleSet -> {
+            for (Entry<T, JavaModuleDescriptor> entry : moduleSet.entrySet()) {
+                if (entry.getValue() != null
+                        && requiredNamedModules.contains(entry.getValue().name())) {
+                    if (collectedModules.add(entry.getValue().name())) {
+                        result.getModulepathElements()
+                                .put(
+                                        entry.getKey(),
+                                        moduleNameSources.get(entry.getValue().name()));
+                        // if the module is an automatic module, add it to the classpath
+                    } else if (entry.getValue().isAutomatic()) {
+                        result.getClasspathElements().add(entry.getKey());
+                    }
+                } else {
+                    result.getClasspathElements().add(entry.getKey());
                 }
-            } else {
-                result.getClasspathElements().add(entry.getKey());
             }
-        }
+        };
+
+        // process defined modules first. This fixes a corner case where a project creates
+        // a main artifact that is a JPMS module and a tests artifact that is not. If the
+        // main artifact and the test artifact happen to have the same module id (one from
+        // the module-info, one from the automatic module naming), the main artifact will
+        // be on the module path and the test artifact will be on the class path.
+        moduleAcceptor.accept(definedModules);
+        moduleAcceptor.accept(automaticModules);
 
         return result;
     }
diff --git a/plexus-java/src/test/java/org/codehaus/plexus/languages/java/jpms/LocationManagerTest.java b/plexus-java/src/test/java/org/codehaus/plexus/languages/java/jpms/LocationManagerTest.java
index acd825d..e9b0c90 100644
--- a/plexus-java/src/test/java/org/codehaus/plexus/languages/java/jpms/LocationManagerTest.java
+++ b/plexus-java/src/test/java/org/codehaus/plexus/languages/java/jpms/LocationManagerTest.java
@@ -161,6 +161,32 @@ void testIdenticalModuleNames() throws Exception {
         ResolvePathsRequest<Path> request =
                 ResolvePathsRequest.ofPaths(Arrays.asList(pj1, pj2)).setMainModuleDescriptor(mockModuleInfoJava);
 
+        when(asmParser.getModuleDescriptor(pj1))
+                .thenReturn(JavaModuleDescriptor.newModule("plexus.java").build());
+        when(asmParser.getModuleDescriptor(pj2))
+                .thenReturn(JavaModuleDescriptor.newModule("plexus.java").build());
+
+        ResolvePathsResult<Path> result = locationManager.resolvePaths(request);
+
+        assertThat(result.getMainModuleDescriptor()).isEqualTo(descriptor);
+        assertThat(result.getPathElements()).hasSize(2);
+        assertThat(result.getModulepathElements()).hasSize(1);
+        assertThat(result.getModulepathElements()).containsKey(pj1);
+        assertThat(result.getModulepathElements()).doesNotContainKey(pj2);
+        assertThat(result.getClasspathElements()).isEmpty();
+        assertThat(result.getPathExceptions()).isEmpty();
+    }
+
+    @Test
+    public void testIdenticalAutomaticModuleNames() throws Exception {
+        Path pj1 = Paths.get("src/test/resources/jar.empty/plexus-java-1.0.0-SNAPSHOT.jar");
+        Path pj2 = Paths.get("src/test/resources/jar.empty.2/plexus-java-2.0.0-SNAPSHOT.jar");
+        JavaModuleDescriptor descriptor =
+                JavaModuleDescriptor.newModule("base").requires("plexus.java").build();
+        when(qdoxParser.fromSourcePath(any(Path.class))).thenReturn(descriptor);
+        ResolvePathsRequest<Path> request =
+                ResolvePathsRequest.ofPaths(Arrays.asList(pj1, pj2)).setMainModuleDescriptor(mockModuleInfoJava);
+
         when(asmParser.getModuleDescriptor(pj1))
                 .thenReturn(
                         JavaModuleDescriptor.newAutomaticModule("plexus.java").build());
@@ -173,8 +199,35 @@ void testIdenticalModuleNames() throws Exception {
         assertThat(result.getPathElements()).hasSize(2);
         assertThat(result.getModulepathElements()).containsOnlyKeys(pj1);
         assertThat(result.getModulepathElements()).doesNotContainKey(pj2);
-        assertThat(result.getClasspathElements()).hasSize(0);
-        assertThat(result.getPathExceptions()).hasSize(0);
+        assertThat(result.getClasspathElements()).hasSize(1);
+        assertThat(result.getPathExceptions()).isEmpty();
+    }
+
+    @Test
+    public void testMainJarModuleAndTestJarAutomatic() throws Exception {
+        Path pj1 = Paths.get("src/test/resources/jar.tests/plexus-java-1.0.0-SNAPSHOT.jar");
+        Path pj2 = Paths.get("src/test/resources/jar.tests/plexus-java-1.0.0-SNAPSHOT-tests.jar");
+        JavaModuleDescriptor descriptor =
+                JavaModuleDescriptor.newModule("base").requires("plexus.java").build();
+        when(qdoxParser.fromSourcePath(any(Path.class))).thenReturn(descriptor);
+        ResolvePathsRequest<Path> request =
+                ResolvePathsRequest.ofPaths(Arrays.asList(pj1, pj2)).setMainModuleDescriptor(mockModuleInfoJava);
+
+        when(asmParser.getModuleDescriptor(pj1))
+                .thenReturn(JavaModuleDescriptor.newModule("plexus.java").build());
+        when(asmParser.getModuleDescriptor(pj2)).thenReturn(null);
+
+        ResolvePathsResult<Path> result = locationManager.resolvePaths(request);
+
+        assertThat(result.getMainModuleDescriptor()).isEqualTo(descriptor);
+        assertThat(result.getPathElements()).hasSize(2);
+        assertThat(result.getModulepathElements()).hasSize(1);
+        assertThat(result.getModulepathElements()).containsKey(pj1);
+        assertThat(result.getModulepathElements()).doesNotContainKey(pj2);
+        assertThat(result.getClasspathElements()).hasSize(1);
+        assertThat(result.getClasspathElements()).contains(pj2);
+        assertThat(result.getClasspathElements()).doesNotContain(pj1);
+        assertThat(result.getPathExceptions()).isEmpty();
     }
 
     @Test
diff --git a/plexus-java/src/test/resources/jar.tests/plexus-java-1.0.0-SNAPSHOT-tests.jar b/plexus-java/src/test/resources/jar.tests/plexus-java-1.0.0-SNAPSHOT-tests.jar
new file mode 100644
index 0000000..a6afd0f
Binary files /dev/null and b/plexus-java/src/test/resources/jar.tests/plexus-java-1.0.0-SNAPSHOT-tests.jar differ
diff --git a/plexus-java/src/test/resources/jar.tests/plexus-java-1.0.0-SNAPSHOT.jar b/plexus-java/src/test/resources/jar.tests/plexus-java-1.0.0-SNAPSHOT.jar
new file mode 100644
index 0000000..413ae96
Binary files /dev/null and b/plexus-java/src/test/resources/jar.tests/plexus-java-1.0.0-SNAPSHOT.jar differ