Remove duplicate Eclipse .classpath entries in Gradle 5.6+

The workaround used in this commit was authored by @marcphilipp for
gradle/gradle#10393.

buildSrc/src/main/kotlin/java-library-conventions.gradle.kts

@@ -1,3 +1,5 @@
+import org.gradle.plugins.ide.eclipse.model.Classpath
+
 plugins {
 	`java-library`
 	eclipse
@@ -31,6 +33,18 @@ sourceSets {
 eclipse {
 	classpath {
 		plusConfigurations.add(shadowed)
+
+		// Workaround for https://github.com/gradle/gradle/issues/10393
+		file {
+			whenMerged(Action<Classpath> {
+				entries = entries.distinctBy {
+					if (it is org.gradle.plugins.ide.eclipse.model.ProjectDependency)
+						it.path
+					else
+						it
+				}
+			})
+		}
 	}
 }