diff --git a/owlps-positioner/posexcept.cc b/owlps-positioner/posexcept.cc
index 79e8813..f0346b7 100644
--- a/owlps-positioner/posexcept.cc
+++ b/owlps-positioner/posexcept.cc
@@ -28,6 +28,11 @@ cannot_merge(const string &_explanation) throw():
   posexcept(_explanation) {}
 
 
+element_already_exists::
+element_already_exists(const string &_explanation) throw():
+  posexcept("Element already exists in collection: " + _explanation) {}
+
+
 element_not_found::
 element_not_found(const string &_explanation) throw():
   posexcept("Element not found in collection: " + _explanation) {}
diff --git a/owlps-positioner/posexcept.hh b/owlps-positioner/posexcept.hh
index 3cc021e..d9b67e9 100644
--- a/owlps-positioner/posexcept.hh
+++ b/owlps-positioner/posexcept.hh
@@ -50,6 +50,13 @@ public:
 } ;
 
 
+class element_already_exists: public posexcept
+{
+public:
+  element_already_exists(const std::string &_explanation) throw() ;
+} ;
+
+
 class element_not_found: public posexcept
 {
 public: