diff --git a/lib/base/array.cpp b/lib/base/array.cpp
index 9d236954b..d9e8ea06c 100644
--- a/lib/base/array.cpp
+++ b/lib/base/array.cpp
@@ -108,6 +108,20 @@ size_t Array::GetLength(void) const
 	return m_Data.size();
 }
 
+/**
+ * Checks whether the array contains the specified value.
+ *
+ * @param value The value.
+ * @returns true if the array contains the value, false otherwise.
+ */
+bool Array::Contains(const String& value) const
+{
+	ASSERT(!OwnsLock());
+	ObjectLock olock(this);
+
+	return (std::find(m_Data.begin(), m_Data.end(), value) != m_Data.end());
+}
+
 /**
  * Insert the given value at the specified index
  *
diff --git a/lib/base/array.h b/lib/base/array.h
index 0e53fbf78..b7e39b0ef 100644
--- a/lib/base/array.h
+++ b/lib/base/array.h
@@ -50,6 +50,7 @@ public:
 	Iterator End(void);
 
 	size_t GetLength(void) const;
+	bool Contains(const String& value) const;
 
 	void Insert(unsigned int index, const Value& value);
 	void Remove(unsigned int index);
diff --git a/lib/icinga/checkable.cpp b/lib/icinga/checkable.cpp
index eabce35e3..d60ffaf44 100644
--- a/lib/icinga/checkable.cpp
+++ b/lib/icinga/checkable.cpp
@@ -94,6 +94,9 @@ void Checkable::AddGroup(const String& name)
 
 	Array::Ptr groups = GetGroups();
 
+	if (groups && groups->Contains(name))
+		return;
+
 	if (!groups)
 		groups = make_shared<Array>();
 
diff --git a/lib/icinga/user.cpp b/lib/icinga/user.cpp
index ca29c2935..71ab946b8 100644
--- a/lib/icinga/user.cpp
+++ b/lib/icinga/user.cpp
@@ -75,6 +75,9 @@ void User::AddGroup(const String& name)
 
 	Array::Ptr groups = GetGroups();
 
+	if (groups && groups->Contains(name))
+		return;
+
 	if (!groups)
 		groups = make_shared<Array>();