Fix duplicate responses in XPath following, following-sibling, preceding, preceding-sibling

## Why?
See: #251 (comment)

- XPath : a/d/preceding::* => ["d", "c", "b"]
```xml
<a>
  <b/> <!-- a/d/preceding::b -->
  <c/> <!-- a/d/preceding::c -->
  <d/> <!-- a/d/preceding::d -->
  <d/> <!-- self -->
  <e/>
  <f/>
</a>
```

- XPath : a/d/following::* => ["d", "e", "f"]
```xml
<a>
  <b/>
  <c/>
  <d/> <!-- self -->
  <d/> <!-- a/d/following::d -->
  <e/> <!-- a/d/following::e -->
  <f/> <!-- a/d/following::f -->
</a>
```

- XPath : a/b/x/following-sibling:* => ["c", "d", "e"]
```xml
<a>
  <b>
    <x/> <!-- self -->
    <c/> <!-- a/b/x/following-sibling::c -->
    <d/> <!-- a/b/x/following-sibling::d -->
 </b>
  <b>
    <x/> <!-- self -->
    <e/> <!-- a/b/x/following-sibling::e -->
  </b>
</a>
```

- XPath : a/b/x/following-sibling:* => ["c", "d", "x", "e"]
```xml
<a>
  <b>
    <x/> <!-- self -->
    <c/> <!-- a/b/x/following-sibling::c -->
    <d/> <!-- a/b/x/following-sibling::d -->
    <x/> <!-- a/b/x/following-sibling::x -->
    <e/> <!-- a/b/x/following-sibling::e -->
  </b>
</a>
```

- XPath : a/b/x/preceding-sibling::* => ["e", "d", "c"]
```xml
<a>
  <b>
    <c/>  <!-- a/b/x/preceding-sibling::c -->
    <d/>  <!-- a/b/x/preceding-sibling::d -->
    <x/>  <!-- self -->
  </b>
  <b>
    <e/>  <!-- a/b/x/preceding-sibling::e -->
    <x/>  <!-- self -->
  </b>
</a>
```

- XPath : a/b/x/preceding-sibling::* => ["e", "x", "d", "c"]
```xml
<a>
  <b>
    <c/>  <!-- a/b/x/preceding-sibling::c -->
    <d/>  <!-- a/b/x/preceding-sibling::d -->
    <x/>  <!-- a/b/x/preceding-sibling::x -->
    <e/>  <!-- a/b/x/preceding-sibling::e -->
    <x/>  <!-- self -->
  </b>
</a>
```

- XPath : //a/following-sibling:*[1] => ["w", "x", "y", "z"]
```xml
<div>
  <div>
    <a/> <-- self -->
    <w/> <-- //a/following-sibling:*[1] -->
  </div>
  <a/> <-- self -->
  <x/> <-- //a/following-sibling:*[1] -->
  <a/> <-- self -->
  <y/> <-- //a/following-sibling:*[1] -->
  <a/> <-- self -->
  <z/> <-- //a/following-sibling:*[1] -->
</div>
```

Author: naitoh

lib/rexml/xpath_parser.rb

@@ -144,7 +144,7 @@ def match(path_stack, nodeset)
       result = expr(path_stack, nodeset)
       case result
       when Array # nodeset
-        unnode(result)
+        unnode(result).uniq
       else
         [result]
       end

test/xpath/test_base.rb

@@ -416,12 +416,103 @@ def test_preceding
       assert_equal( 4, cs.length )
     end
 
-    def test_preceding_sibling
-      d = REXML::Document.new("<a><b><c/><d/><x/></b><b><e/><x/></b></a>")
-      matches = REXML::XPath.match(d, "a/b/x/preceding-sibling::node()")
+    def test_preceding_multiple
+      source = <<-XML
+<a>
+  <b/><c/><d/><d/><e/><f/>
+</a>
+      XML
+      doc = REXML::Document.new(source)
+      matches = REXML::XPath.match(doc, "a/d/preceding::*")
+      assert_equal(["d", "c", "b"], matches.map(&:name))
+    end
+
+    def test_following_multiple
+      source = <<-XML
+<a>
+  <b/><c/><d/><d/><e/><f/>
+</a>
+      XML
+      doc = REXML::Document.new(source)
+      matches = REXML::XPath.match(doc, "a/d/following::*")
+      assert_equal(["d", "e", "f"], matches.map(&:name))
+    end
+
+    def test_following_sibling_across_multiple_nodes
+      source = <<-XML
+<a>
+  <b>
+    <x/><c/><d/>
+  </b>
+  <b>
+    <x/><e/>
+  </b>
+</a>
+      XML
+      doc = REXML::Document.new(source)
+      matches = REXML::XPath.match(doc, "a/b/x/following-sibling::*")
+      assert_equal(["c", "d", "e"], matches.map(&:name))
+    end
+
+    def test_following_sibling_within_single_node
+      source = <<-XML
+<a>
+  <b>
+    <x/><c/><d/><x/><e/>
+  </b>
+</a>
+      XML
+      doc = REXML::Document.new(source)
+      matches = REXML::XPath.match(doc, "a/b/x/following-sibling::*")
+      assert_equal(["c", "d", "x", "e"], matches.map(&:name))
+    end
+
+    def test_following_sibling_predicates
+      source = <<-XML
+<div>
+  <div>
+    <a/><w/>
+  </div>
+  <a/><x/>
+  <a/><y/>
+  <a/><z/>
+</div>
+      XML
+      doc = REXML::Document.new(source)
+      # Finds a node flowing <a/>
+      matches = REXML::XPath.match(doc, "//a/following-sibling::*[1]")
+      assert_equal(["w", "x", "y", "z"], matches.map(&:name))
+    end
+
+    def test_preceding_sibling_across_multiple_nodes
+      source = <<-XML
+<a>
+  <b>
+    <c/><d/><x/>
+  </b>
+  <b>
+    <e/><x/>
+  </b>
+</a>
+      XML
+      doc = REXML::Document.new(source)
+      matches = REXML::XPath.match(doc, "a/b/x/preceding-sibling::*")
       assert_equal(["e", "d", "c"], matches.map(&:name))
     end
 
+    def test_preceding_sibling_within_single_node
+      source = <<-XML
+<a>
+  <b>
+    <c/><d/><x/><e/><x/>
+  </b>
+</a>
+      XML
+      doc = REXML::Document.new(source)
+      matches = REXML::XPath.match(doc, "a/b/x/preceding-sibling::*")
+      assert_equal(["e", "x", "d", "c"], matches.map(&:name))
+    end
+
     def test_following
       d = Document.new "<a><b id='0'/><b/><b><c id='1'/><c id='2'/></b><b id='1'/></a>"
       start = XPath.first( d, "/a/b[@id='0']" )