diff --git a/plugins/org.eclipse.glsp.server/src/org/eclipse/glsp/server/di/ServerModule.java b/plugins/org.eclipse.glsp.server/src/org/eclipse/glsp/server/di/ServerModule.java
index fcdf5292..3fe57e37 100644
--- a/plugins/org.eclipse.glsp.server/src/org/eclipse/glsp/server/di/ServerModule.java
+++ b/plugins/org.eclipse.glsp.server/src/org/eclipse/glsp/server/di/ServerModule.java
@@ -86,7 +86,7 @@ public class ServerModule extends GLSPModule {
     */
    public ServerModule configureDiagramModule(final DiagramModule diagramModule, final Module... mixinModules) {
       String diagramType = diagramModule.getDiagramType();
-      if (!diagramModules.containsKey(diagramType)) {
+      if (diagramModules.containsKey(diagramType)) {
          throw new IllegalArgumentException(
             "A module configuration is already present for diagram type: " + diagramType);
       }
diff --git a/plugins/org.eclipse.glsp.server/src/org/eclipse/glsp/server/utils/BiIndex.java b/plugins/org.eclipse.glsp.server/src/org/eclipse/glsp/server/utils/BiIndex.java
index c2ff95c2..832bf30b 100644
--- a/plugins/org.eclipse.glsp.server/src/org/eclipse/glsp/server/utils/BiIndex.java
+++ b/plugins/org.eclipse.glsp.server/src/org/eclipse/glsp/server/utils/BiIndex.java
@@ -32,10 +32,16 @@ public class BiIndex<K, V> {
    protected Map<K, V> map = new HashMap<>();
    protected Map<V, K> inverseMap = new HashMap<>();
 
+   /**
+    * Returns an unmodifiable map of the currently indexed key-value pairs.
+    */
    public Map<K, V> map() {
       return Map.copyOf(map);
    }
 
+   /**
+    * Returns an unmodifiable inversed map (value to key) of the currently indexed key-value pairs.
+    */
    public Map<V, K> inverseMap() {
       return Map.copyOf(inverseMap);
    }