Ensure buildSrc is only added once to eclipse

Author: philwebb

build.gradle

@@ -661,8 +661,11 @@ configure(rootProject) {
 	apply from: "${gradleScriptDir}/jdiff.gradle"
 
 	eclipse.classpath.file.whenMerged { classpath ->
-		classpath.entries.add(0, new org.gradle.plugins.ide.eclipse.model.SourceFolder(
-			"buildSrc/src/main/groovy", null))
+		def sourceFolder = new org.gradle.plugins.ide.eclipse.model.SourceFolder(
+			"buildSrc/src/main/groovy", null)
+		if(!classpath.entries.contains(sourceFolder)) {
+			classpath.entries.add(0, sourceFolder)
+		}
 	}
 
 	reference {