diff --git a/src/main/resources/com/adobe/epubcheck/schema/20/sch/opf.sch b/src/main/resources/com/adobe/epubcheck/schema/20/sch/opf.sch
index 73a2759a4..9b5043bbe 100644
--- a/src/main/resources/com/adobe/epubcheck/schema/20/sch/opf.sch
+++ b/src/main/resources/com/adobe/epubcheck/schema/20/sch/opf.sch
@@ -1,14 +1,27 @@
-<sch:schema xmlns:sch="http://www.ascc.net/xml/schematron">
+<sch:schema xmlns:sch="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
 
   <sch:ns prefix="dc" uri="http://purl.org/dc/elements/1.1/"/>
   <sch:ns prefix="opf" uri="http://www.idpf.org/2007/opf"/>
 
-  <sch:pattern name="opf_idAttrUnique" id="opf_idAttrUnique">
+  <sch:pattern id="opf_idAttrUnique">
     <!-- id attribute value must be unique for any id attribute in opf file-->
     <sch:rule context="//*[@id]">
       <sch:assert test="count(//@id[normalize-space(.) = normalize-space(current()/@id)]) = 1">The
         "id" attribute does not have a unique value</sch:assert>
     </sch:rule>
   </sch:pattern>
+  
+  <sch:pattern id="opf_guideReferenceUnique">
+    <!-- guide/reference element should be unique (#493) -->
+    <sch:rule context="opf:reference">
+      <sch:let name="current_type_normalized" value="normalize-space(lower-case(@type))"/>
+      <sch:let name="current_href_normalized" value="normalize-space(lower-case(@href))"/>
+      <sch:assert test="
+        count(//opf:reference[
+          normalize-space(lower-case(@type)) = $current_type_normalized and
+          normalize-space(lower-case(@href)) = $current_href_normalized
+        ]) = 1">WARNING: Duplicate 'reference' elements with the same 'type' and 'href' attributes</sch:assert>
+    </sch:rule>
+  </sch:pattern>
 
 </sch:schema>
diff --git a/src/main/resources/com/adobe/epubcheck/schema/30/package-30.sch b/src/main/resources/com/adobe/epubcheck/schema/30/package-30.sch
index af3d66e98..664a78d10 100644
--- a/src/main/resources/com/adobe/epubcheck/schema/30/package-30.sch
+++ b/src/main/resources/com/adobe/epubcheck/schema/30/package-30.sch
@@ -281,6 +281,19 @@
         </rule>
     </pattern>
     
+    <pattern id="opf_guideReferenceUnique">
+        <!-- guide/reference element should be unique (#493) -->
+        <rule context="opf:reference">
+            <let name="current_type_normalized" value="normalize-space(lower-case(@type))"/>
+            <let name="current_href_normalized" value="normalize-space(lower-case(@href))"/>
+            <assert test="
+                count(//opf:reference[
+                    normalize-space(lower-case(@type)) = $current_type_normalized and
+                    normalize-space(lower-case(@href)) = $current_href_normalized
+                ]) = 1">WARNING: Duplicate 'reference' elements with the same 'type' and 'href' attributes</assert>
+        </rule>
+    </pattern>
+    
     <include href="./mod/id-unique.sch"/>
 
 </schema>
diff --git a/src/test/java/com/adobe/epubcheck/opf/OPFCheckerTest.java b/src/test/java/com/adobe/epubcheck/opf/OPFCheckerTest.java
index b22d0e905..95c1c8c1d 100644
--- a/src/test/java/com/adobe/epubcheck/opf/OPFCheckerTest.java
+++ b/src/test/java/com/adobe/epubcheck/opf/OPFCheckerTest.java
@@ -845,6 +845,20 @@ public void testIDX_CollectionInvalid_ResourceNotContentDoc()
     testValidateDocument("invalid/idx-collection-resource-noxhtml.opf", EPUBVersion.VERSION_3);
   }
 
+  @Test
+  public void testGuideReferenceUnique_EPUB2()
+  {
+    Collections.addAll(expectedWarnings, MessageId.RSC_017, MessageId.RSC_017);
+    testValidateDocument("invalid/guide-duplicates.opf", EPUBVersion.VERSION_2);
+  }
+
+  @Test
+  public void testGuideReferenceUnique_EPUB3()
+  {
+    Collections.addAll(expectedWarnings, MessageId.RSC_017, MessageId.RSC_017);
+    testValidateDocument("invalid/guide-duplicates.opf", EPUBVersion.VERSION_3);
+  }
+
   @Test
   public void testDict_Single()
   {
diff --git a/src/test/resources/20/single/opf/invalid/guide-duplicates.opf b/src/test/resources/20/single/opf/invalid/guide-duplicates.opf
new file mode 100644
index 000000000..6366ec19b
--- /dev/null
+++ b/src/test/resources/20/single/opf/invalid/guide-duplicates.opf
@@ -0,0 +1,25 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<package xmlns:opf="http://www.idpf.org/2007/opf" xmlns:dcterms="http://purl.org/dc/terms/" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.idpf.org/2007/opf" version="2.0" unique-identifier="id">
+  <metadata>
+    <dc:rights>Public domain in the USA.</dc:rights>
+    <dc:identifier id="id" opf:scheme="URI">http://www.gutenberg.org/ebooks/36582</dc:identifier>
+    <dc:creator opf:file-as="Anonymous">Anonymous</dc:creator>
+    <dc:title>Advice to Sunday School Children</dc:title>
+    <dc:language xsi:type="dcterms:RFC4646">en</dc:language>
+    <dc:date opf:event="publication">2011-07-02</dc:date>
+    <dc:date opf:event="conversion">2011-07-03T19:26:31.417584+00:00</dc:date>
+    <dc:source>http://www.gutenberg.org/files/36582/36582-h/36582-h.htm</dc:source>
+  </metadata>
+  <manifest>
+    <item href="pgepub.css" id="item12" media-type="text/css"/>
+    <item href="toc.ncx" id="ncx" media-type="application/x-dtbncx+xml"/>
+    <item href="wrap0000.html" id="coverpage-wrapper" media-type="application/xhtml+xml"/>
+  </manifest>
+  <spine toc="ncx">
+    <itemref idref="coverpage-wrapper"/>
+  </spine>
+  <guide>
+    <reference href="wrap0000.html" type="cover" title="Cover"/>
+    <reference href="wrap0000.html" type="cover" title="Cover"/>
+  </guide>
+</package>
diff --git a/src/test/resources/30/single/opf/invalid/guide-duplicates.opf b/src/test/resources/30/single/opf/invalid/guide-duplicates.opf
new file mode 100644
index 000000000..7b521d095
--- /dev/null
+++ b/src/test/resources/30/single/opf/invalid/guide-duplicates.opf
@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<?oxygen RNGSchema="../../../src/schema/package-30.rnc" type="compact"?>
+<?oxygen SCHSchema="../../../src/schema/package-30.sch"?>
+<package xmlns="http://www.idpf.org/2007/opf" version="3.0" unique-identifier="uid" xmlns:dc="http://purl.org/dc/elements/1.1/">
+    <metadata>
+        <dc:identifier id="uid">urn:uuid:550e8412-e29b-41d4-a716-446655441234</dc:identifier>
+        <dc:title>Norwegian Wood</dc:title>        
+        <dc:language>en</dc:language>
+        <meta property="dcterms:modified">2011-08-19T12:00:00Z</meta>
+    </metadata> 
+    <manifest>
+        <item id="t001" href="contents.xhtml" properties="nav" media-type="application/xhtml+xml" />        
+    </manifest>
+    <spine>
+        <itemref idref="t001" />
+    </spine>
+    <guide>
+        <reference href="contents.xhtml" type="cover" title="Cover"/>
+        <reference href="cover.xhtml" type="cover" title="Cover"/>
+        <reference href="contents.xhtml" type="cover"/>
+    </guide>
+</package>