diff --git a/compile/inc/Incremental.scala b/compile/inc/Incremental.scala
index e6e8be492..211db2ce1 100644
--- a/compile/inc/Incremental.scala
+++ b/compile/inc/Incremental.scala
@@ -87,10 +87,19 @@ object Incremental
 
 	def invalidateIncremental(previous: Relations, changes: APIChanges[File], recompiledSources: Set[File], log: Logger): Set[File] =
 	{
-		val inv = invalidateTransitive(previous.usesInternalSrc _,  changes.modified, log)// ++ scopeInvalidations(previous.extAPI _, changes.modified, changes.names)
+		val inv =
+			invalidateTransitive(previous.usesInternalSrc _,  changes.modified, log) ++
+			invalidateDuplicates(previous)
+			// ++ scopeInvalidations(previous.extAPI _, changes.modified, changes.names)
 		if((inv -- recompiledSources).isEmpty) Set.empty else inv
 	}
 
+	/** Invalidate all sources that claim to produce the same class file as another source file. */
+	def invalidateDuplicates(merged: Relations): Set[File] =
+		merged.srcProd.reverseMap.flatMap { case (classFile, sources) =>
+			if(sources.size > 1) sources else Nil
+		} toSet;
+
 	/** Only invalidates direct source dependencies.  It excludes any sources that were recompiled during the previous run.
 	* Callers may want to augment the returned set with 'modified' or all sources recompiled up to this point. */
 	def invalidateDirect(dependsOnSrc: File => Set[File], modified: Set[File]): Set[File] =
diff --git a/sbt/src/sbt-test/source-dependencies/dup-class/changes/A.scala b/sbt/src/sbt-test/source-dependencies/dup-class/changes/A.scala
new file mode 100644
index 000000000..69c493db2
--- /dev/null
+++ b/sbt/src/sbt-test/source-dependencies/dup-class/changes/A.scala
@@ -0,0 +1 @@
+object A
diff --git a/sbt/src/sbt-test/source-dependencies/dup-class/changes/A2.scala b/sbt/src/sbt-test/source-dependencies/dup-class/changes/A2.scala
new file mode 100644
index 000000000..69c493db2
--- /dev/null
+++ b/sbt/src/sbt-test/source-dependencies/dup-class/changes/A2.scala
@@ -0,0 +1 @@
+object A
diff --git a/sbt/src/sbt-test/source-dependencies/dup-class/changes/B.scala b/sbt/src/sbt-test/source-dependencies/dup-class/changes/B.scala
new file mode 100644
index 000000000..251ef7397
--- /dev/null
+++ b/sbt/src/sbt-test/source-dependencies/dup-class/changes/B.scala
@@ -0,0 +1 @@
+object B
diff --git a/sbt/src/sbt-test/source-dependencies/dup-class/test b/sbt/src/sbt-test/source-dependencies/dup-class/test
new file mode 100644
index 000000000..531553bb5
--- /dev/null
+++ b/sbt/src/sbt-test/source-dependencies/dup-class/test
@@ -0,0 +1,6 @@
+$ copy-file changes/A.scala A.scala
+> compile
+$ copy-file changes/A2.scala B.scala
+-> compile
+$ copy-file changes/B.scala B.scala
+> compile