diff --git a/engine/runtime/src/main/java/org/enso/interpreter/runtime/error/RedefinedConstructorException.java b/engine/runtime/src/main/java/org/enso/interpreter/runtime/error/RedefinedConstructorException.java
new file mode 100644
index 000000000000..51e4859fb9a6
--- /dev/null
+++ b/engine/runtime/src/main/java/org/enso/interpreter/runtime/error/RedefinedConstructorException.java
@@ -0,0 +1,17 @@
+package org.enso.interpreter.runtime.error;
+
+import com.oracle.truffle.api.exception.AbstractTruffleException;
+import org.enso.interpreter.runtime.Module;
+
+/** An exception thrown when the program tries to redefine an already-defined constructor */
+public class RedefinedConstructorException extends AbstractTruffleException {
+
+  /**
+   * Creates a new error.
+   *
+   * @param atom the method of the atom on which {@code method} is being defined
+   */
+  public RedefinedConstructorException(String atom, Module scope) {
+    super("Constructors cannot be overloaded, but you have tried to overload " + atom + " in scope " + scope.getName());
+  }
+}
diff --git a/engine/runtime/src/main/java/org/enso/interpreter/runtime/scope/ModuleScope.java b/engine/runtime/src/main/java/org/enso/interpreter/runtime/scope/ModuleScope.java
index 88a5194da210..ad52574e7202 100644
--- a/engine/runtime/src/main/java/org/enso/interpreter/runtime/scope/ModuleScope.java
+++ b/engine/runtime/src/main/java/org/enso/interpreter/runtime/scope/ModuleScope.java
@@ -8,6 +8,7 @@
 import org.enso.interpreter.runtime.Module;
 import org.enso.interpreter.runtime.callable.atom.AtomConstructor;
 import org.enso.interpreter.runtime.callable.function.Function;
+import org.enso.interpreter.runtime.error.RedefinedConstructorException;
 import org.enso.interpreter.runtime.error.RedefinedMethodException;
 import org.enso.interpreter.runtime.error.RedefinedConversionException;
 
@@ -38,9 +39,17 @@ public ModuleScope(Module module) {
    * @param constructor the constructor to register
    */
   public void registerConstructor(AtomConstructor constructor) {
+    var name = constructor.getName();
+    if (constructors.containsKey(name))
+      throw new RedefinedConstructorException(name, module);
+    constructors.put(name, constructor);
+  }
+
+  public void forciblyRegisterConstructor(AtomConstructor constructor) {
     constructors.put(constructor.getName(), constructor);
   }
 
+
   /** @return the associated type of this module. */
   public AtomConstructor getAssociatedType() {
     return associatedType;
diff --git a/engine/runtime/src/main/scala/org/enso/compiler/codegen/RuntimeStubsGenerator.scala b/engine/runtime/src/main/scala/org/enso/compiler/codegen/RuntimeStubsGenerator.scala
index 4c023e7695f8..0c447691a78e 100644
--- a/engine/runtime/src/main/scala/org/enso/compiler/codegen/RuntimeStubsGenerator.scala
+++ b/engine/runtime/src/main/scala/org/enso/compiler/codegen/RuntimeStubsGenerator.scala
@@ -23,13 +23,13 @@ class RuntimeStubsGenerator(builtins: Builtins) {
       BindingAnalysis,
       "Non-parsed module used in stubs generator"
     )
-    val constructors = localBindings.constructors.map { tp =>
+    val constructors = localBindings.constructors./*distinctBy(_.name). */map { tp =>
       if (tp.builtinType) {
         val builtinType = builtins.getBuiltinType(tp.name)
         if (builtinType == null) {
           throw new CompilerError("Unknown @BuiltinType " + tp.name)
         }
-        scope.registerConstructor(builtinType)
+        scope.forciblyRegisterConstructor(builtinType)
         builtinType.setShadowDefinitions(scope)
         (tp, builtinType)
       } else {
diff --git a/engine/runtime/src/main/scala/org/enso/compiler/context/SuggestionBuilder.scala b/engine/runtime/src/main/scala/org/enso/compiler/context/SuggestionBuilder.scala
index ddb67363a1ef..fa5f5b978a4d 100644
--- a/engine/runtime/src/main/scala/org/enso/compiler/context/SuggestionBuilder.scala
+++ b/engine/runtime/src/main/scala/org/enso/compiler/context/SuggestionBuilder.scala
@@ -344,6 +344,13 @@ final class SuggestionBuilder[A: IndexedSource](val source: A) {
     }
   }
 
+  /** Resolve a name to either an explicit sum type or a simple TypeArg. Used to link
+   * suggestions to concrete types, so the editor can know a fully qualified
+   * type name if possible.
+   *
+   * @param resolvedName the name to resolve
+   * @return how to present it to the suggestion engine
+   */
   private def buildResolvedUnionTypeName(
     resolvedName: BindingsMap.ResolvedName
   ): Option[TypeArg] = resolvedName match {