diff --git a/lang/frontend/cli/src/main/java/tools/vitruv/neojoin/cli/Main.java b/lang/frontend/cli/src/main/java/tools/vitruv/neojoin/cli/Main.java
index 0210ec3e..bb5dd836 100644
--- a/lang/frontend/cli/src/main/java/tools/vitruv/neojoin/cli/Main.java
+++ b/lang/frontend/cli/src/main/java/tools/vitruv/neojoin/cli/Main.java
@@ -127,7 +127,11 @@ private void printError(String message, @Nullable Object... args) {
      */
     private int execute() throws IOException {
         // collect available meta-models
-        EPackage.Registry registry = new PackageModelCollector(metaModelPath).collect();
+        var collectionResult = new PackageModelCollector(metaModelPath).collect();
+        List<PackageModelCollector.Issue> issues = collectionResult.left();
+        EPackage.Registry registry = collectionResult.right();
+
+        issues.forEach(System.err::println);
 
         // parse query
         var setup = new NeoJoinStandaloneSetup(registry);
diff --git a/lang/frontend/ide/src/main/java/tools/vitruv/neojoin/ide/Main.java b/lang/frontend/ide/src/main/java/tools/vitruv/neojoin/ide/Main.java
index 0cb9ed70..4a634006 100644
--- a/lang/frontend/ide/src/main/java/tools/vitruv/neojoin/ide/Main.java
+++ b/lang/frontend/ide/src/main/java/tools/vitruv/neojoin/ide/Main.java
@@ -23,6 +23,8 @@
 import java.io.PrintStream;
 import java.io.PrintWriter;
 import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.concurrent.ExecutionException;
 
 @NullUnmarked
@@ -54,12 +56,18 @@ static class Logging {
     public void run() {
         var ioConfig = redirectIo();
 
-        String errorMessage = null;
+        List<String> errorMessages = new ArrayList<>();
         try {
-            NeoJoinIdeSetup.Registry = new PackageModelCollector(metaModelPath).collect();
+            var collectionResult = new PackageModelCollector(metaModelPath).collect();
+            NeoJoinIdeSetup.Registry = collectionResult.right();
+
+            collectionResult.left().forEach(issue ->
+                errorMessages.add(issue.toString())
+            );
         } catch (Exception ex) {
             NeoJoinIdeSetup.Registry = new EPackageRegistryImpl();
-            errorMessage = "Invalid meta-model path: " + ex.getMessage();
+            var errorMessage = "Invalid meta-model path: " + ex.getMessage();
+            errorMessages.add(errorMessage);
             System.err.printf("[ERROR] %s%n  > full meta-model path: %s%n", errorMessage, metaModelPath);
         }
 
@@ -71,9 +79,9 @@ public void run() {
             ioConfig.in(), ioConfig.out(),
             true, ioConfig.trace()
         );
-        if (errorMessage != null) {
-            launcher.getRemoteProxy().showMessage(new MessageParams(MessageType.Error, errorMessage));
-        }
+
+        showErrorMessages(launcher, errorMessages);
+
         languageServer.connect(launcher.getRemoteProxy());
         var future = launcher.startListening();
 
@@ -86,6 +94,12 @@ public void run() {
         }
     }
 
+    private void showErrorMessages(Launcher<LanguageClient> launcher, Iterable<String> errorMessages) {
+        for (var errorMessage : errorMessages) {
+            launcher.getRemoteProxy().showMessage(new MessageParams(MessageType.Error, errorMessage));
+        }
+    }
+
     private record LanguageServerIoConfig(
         InputStream in,
         OutputStream out,
@@ -142,5 +156,4 @@ private boolean isTraceLoggingEnabled() {
     public static void main(String[] args) {
         System.exit(new CommandLine(new Main()).execute(args));
     }
-
 }
diff --git a/lang/frontend/language/src/main/java/tools/vitruv/neojoin/collector/PackageModelCollector.java b/lang/frontend/language/src/main/java/tools/vitruv/neojoin/collector/PackageModelCollector.java
index 68da8758..70ba5710 100644
--- a/lang/frontend/language/src/main/java/tools/vitruv/neojoin/collector/PackageModelCollector.java
+++ b/lang/frontend/language/src/main/java/tools/vitruv/neojoin/collector/PackageModelCollector.java
@@ -6,6 +6,10 @@
 import org.eclipse.emf.ecore.resource.impl.ResourceSetImpl;
 import org.eclipse.emf.ecore.xmi.impl.EcoreResourceFactoryImpl;
 import tools.vitruv.neojoin.utils.EMFUtils;
+import tools.vitruv.neojoin.utils.Pair;
+
+import java.util.ArrayList;
+import java.util.List;
 
 /**
  * Searches for meta-model files with the {@code .ecore} extension and collects them into a package registry.
@@ -25,23 +29,36 @@ public PackageModelCollector(String searchPathString) {
         super(searchPathString);
     }
 
-    public EPackage.Registry collect() {
+    public Pair<List<Issue>, EPackage.Registry> collect() {
         if (!Resource.Factory.Registry.INSTANCE.getExtensionToFactoryMap().containsKey(FileExtension)) {
             Resource.Factory.Registry.INSTANCE.getExtensionToFactoryMap().put(
                 FileExtension, new EcoreResourceFactoryImpl());
         }
 
         var registry = new EPackageRegistryImpl();
+        List<Issue> issues = new ArrayList<>();
 
         collectResourcesAsStream(new ResourceSetImpl()).forEach(res ->
             EMFUtils.getAllEPackages(res).forEach(pack -> {
-                var previousValue = registry.put(pack.getNsURI(), pack);
-                if (previousValue != null) {
-                    throw new IllegalArgumentException("Found multiple packages with URI '%s'.".formatted(pack.getNsURI()));
-            }
-        }));
+                if (registry.containsKey(pack.getNsURI())) {
+                    issues.add(new Issue.PackageURIDuplication(pack.getNsURI()));
+                } else {
+                    registry.put(pack.getNsURI(), pack);
+                }
+            })
+        );
 
-        return registry;
+        return new Pair<>(issues, registry);
     }
 
+    public sealed interface Issue {
+
+        record PackageURIDuplication(String packageUri) implements Issue {
+
+            @Override
+            public String toString() {
+                return "Found multiple packages with URI '%s'.".formatted(packageUri);
+            }
+        }
+    }
 }