[Xamarin.Android.Build.Tasks] _CompileToDalvikWithDx invoked after modifying .cs file in a referenced library project

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=58865

In this case the problem was being caused by the fact that the
AndroidManifest.xml was constantly being modified. The item

	<meta-data android:name="android.support.VERSION" android:value="25.4.0" />

was constantly being duplicated in manifest. As a result it
was always newer and was triggering a change of build targets.

So we need to double check we are NOT adding 100% duplicate
values to the manifest.

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Utilities/ManifestDocument.cs

@@ -394,13 +394,29 @@ void MergeLibraryManifest (string mergedManifest)
 					doc.XPathSelectElement (string.Format ("/manifest/{0}", top.Name.LocalName));
 				if (existing != null)
 					// if there is existing node with the same android:name, then append contents to existing node.
-					existing.Add (FixupNameElements (package, top.Nodes ()));
+					MergeNodesIntoElement (existing, FixupNameElements (package, top.Nodes ()));
 				else
 					// otherwise, just add to the doc.
 					doc.Root.Add (FixupNameElements (package, new XNode [] { top }));
 			}
 		}
 
+		bool AttributesMatch (XElement a, XElement b)
+		{
+			return a.Attributes ().All (x => b.Attribute (x.Name.LocalName).Value == x.Value); ;
+		}
+
+		void MergeNodesIntoElement (XElement element, IEnumerable<XNode> nodes)
+		{
+			var name = AndroidXmlNamespace.GetName ("name");
+			foreach (var node in nodes.Select (x => x as XElement)) {
+				var existing = element.Elements (node.Name.LocalName).Any (x => AttributesMatch (x, node));
+				if (existing)
+					continue;
+				element.Add (node);
+			}
+		}
+
 		IEnumerable<XNode> FixupNameElements(string packageName, IEnumerable<XNode> nodes)
 		{
 			foreach (var element in nodes.Select ( x => x as XElement).Where (x => x != null && ManifestAttributeFixups.ContainsKey (x.Name.LocalName))) {