Added assertion as suggested in JENKINS-22767.

core/src/main/java/jenkins/model/lazy/AbstractLazyLoadRunMap.java

@@ -468,7 +468,8 @@ public abstract class AbstractLazyLoadRunMap<R> extends AbstractMap<Integer,R> i
             Index copy = editInPlace!=null ? editInPlace : new Index(index);
 
             BuildReference<R> ref = createReference(r);
-            copy.byNumber.put(getNumberOf(r),ref);
+            BuildReference<R> old = copy.byNumber.put(getNumberOf(r), ref);
+            assert old == null || old.get() == null : "tried to overwrite " + old + " with " + ref;
 
             if (editInPlace==null)  index = copy;
 

core/src/main/java/jenkins/model/lazy/BuildReference.java

@@ -66,7 +66,7 @@ public final class BuildReference<R> {
         if (this == o) return true;
         if (o == null || getClass() != o.getClass()) return false;
 
-        BuildReference that = (BuildReference) o;
+        BuildReference<?> that = (BuildReference) o;
         return id.equals(that.id);
 
     }
@@ -76,6 +76,11 @@ public final class BuildReference<R> {
         return id.hashCode();
     }
 
+    @Override public String toString() {
+        R r = get();
+        return r != null ? r.toString() : id;
+    }
+
     /**
      * An abstraction of {@link Reference}.
      * @since 1.548