--- a/gwt-client/src/main/java/org/dive4elements/river/client/shared/model/SalixZone.java	Tue Jul 10 14:47:10 2018 +0200
+++ b/gwt-client/src/main/java/org/dive4elements/river/client/shared/model/SalixZone.java	Tue Jul 10 18:53:04 2018 +0200
@@ -110,10 +110,12 @@
         treeList.addAll(list);
         double lowerCompare = lower + DELTA;
         for (final SalixZone zone : treeList) {
-            if (zone.getLowerFromTo() > lowerCompare) {
-                return true;
+            if ((zone.getUpperFromTo() - zone.getLowerFromTo()) > DELTA) {
+                if (zone.getLowerFromTo() > lowerCompare) {
+                    return true;
+                }
+                lowerCompare = zone.getUpperFromTo() + DELTA;
             }
-            lowerCompare = zone.getUpperFromTo() + DELTA;
         }
         if ((lowerCompare + DELTA) < upper)
             return true; // am Ende nicht geschlossen
@@ -149,6 +151,9 @@
     @Override
     public int compareTo(final SalixZone o) {
         final int basicCompare = this.getLowerFromTo().compareTo(o.getLowerFromTo());
+        if (basicCompare == 0) {
+            return 1; // necessary for the treeSet!
+        }
         return basicCompare;
     }