diff --git a/core/lib/Drupal/Core/Cache/StaticCache/LruStaticCache.php b/core/lib/Drupal/Core/Cache/StaticCache/LruStaticCache.php
index 7762c70..e4c7693 100644
--- a/core/lib/Drupal/Core/Cache/StaticCache/LruStaticCache.php
+++ b/core/lib/Drupal/Core/Cache/StaticCache/LruStaticCache.php
@@ -6,9 +6,11 @@
 
 
 /**
- * Defines a static cache implementation.
+ * Defines a least recently used (LRU) static cache implementation.
  *
- * Stores cache items in memory using a PHP array.
+ * Stores cache items in memory using a PHP array. The number of cache items is
+ * limited to a fixed number of slots. When the all slots are full, older items
+ * are purged based on least recent usage.
  *
  * @ingroup cache
  */
@@ -68,16 +70,22 @@ public function get($cid, $allow_invalid = FALSE) {
    * {@inheritdoc}
    */
   public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array()) {
-    // If there are still available slots, decrement the counter.
-    if ($this->availableSlots) {
-      $this->availableSlots--;
-    }
-    // Otherwise remove the least recently used item.
-    else {
+    // If there are no available slots, and none of the slots are already used
+    // by this cache ID, remove the least recently used item.
+    $current_position = array_search($cid, $this->positions);
+    if (!$this->availableSlots && $current_position === FALSE) {
       $key = array_shift($this->positions);
       unset($this->cache[$key]);
+      $this->availableSlots++;
+    }
+    // Setting an already-set item moves it to the most recently used spot but
+    // does not take up any extra slots.
+    if ($current_position !== FALSE) {
+      unset($this->positions[$current_position]);
+    }
+    else {
+      $this->availableSlots--;
     }
-    // Add this item to the most recently used slot.
     $this->positions[] = $cid;
     return parent::set($cid, $data, $expire, $tags);
   }
@@ -86,9 +94,11 @@ public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array
    * {@inheritdoc}
    */
   public function delete($cid) {
-    $this->availableSlots++;
     $current_position = array_search($cid, $this->positions);
-    unset($this->positions[$current_position]);
+    if ($current_position !== FALSE) {
+      $this->availableSlots++;
+      unset($this->positions[$current_position]);
+    }
     parent::delete($cid);
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Cache/LruStaticCacheTest.php b/core/tests/Drupal/Tests/Core/Cache/LruStaticCacheTest.php
index 9c8fdb7..d4e93d0 100644
--- a/core/tests/Drupal/Tests/Core/Cache/LruStaticCacheTest.php
+++ b/core/tests/Drupal/Tests/Core/Cache/LruStaticCacheTest.php
@@ -44,6 +44,29 @@ public function testGetSetDelete() {
     $cids[2] = ['crow', FALSE];
     $cids[5] = ['bigger_cuckoo', 'bigger_cuckoo'];
     $this->assertCids($cids);
+
+    // Confirm that setting the same item multiple times only uses one slot.
+    $this->staticCache->set('bigger_cuckoo', 'bigger_cuckoo');
+    $this->staticCache->set('bigger_cuckoo', 'bigger_cuckoo');
+    $this->staticCache->set('bigger_cuckoo', 'bigger_cuckoo');
+    $this->staticCache->set('bigger_cuckoo', 'bigger_cuckoo');
+    $this->staticCache->set('bigger_cuckoo', 'bigger_cuckoo');
+    $this->assertCids($cids);
+
+    // Confirm that deleting the same item multiple times only frees up one
+    // slot.
+    $this->staticCache->delete('bigger_cuckoo', 'bigger_cuckoo');
+    $this->staticCache->delete('bigger_cuckoo', 'bigger_cuckoo');
+    $this->staticCache->delete('bigger_cuckoo', 'bigger_cuckoo');
+    $this->staticCache->delete('bigger_cuckoo', 'bigger_cuckoo');
+    $this->staticCache->delete('bigger_cuckoo', 'bigger_cuckoo');
+    $this->staticCache->delete('bigger_cuckoo', 'bigger_cuckoo');
+    $this->staticCache->set('bigger_cuckoo', 'bigger_cuckoo');
+    $this->assertCids($cids);
+    $this->staticCache->set('crow', 'crow');
+    $cids[2] = ['crow', 'crow'];
+    $cids[1] = ['pidgie', FALSE];
+    $this->assertCids($cids);
   }
 
   public function testInvalidate() {