diff --git a/akka-stream-utils/jvm/src/test/scala/StreamOpsSpec.scala b/akka-stream-utils/jvm/src/test/scala/StreamOpsSpec.scala
index 45a7191..92e5eba 100644
--- a/akka-stream-utils/jvm/src/test/scala/StreamOpsSpec.scala
+++ b/akka-stream-utils/jvm/src/test/scala/StreamOpsSpec.scala
@@ -242,12 +242,18 @@ class StreamOpsSpec extends ScalaTestWithActorTestKit with AnyWordSpecLike with
 
       checkTable2[Boolean, Int, Either](input, operations)
     }
-    "Either flattenF" in {
+    "Either collectRightF" in {
       Source(Seq(Right(1), Left(true), Right(2)))
-        .flattenF
+        .collectRightF
         .runWith(Sink.seq)
         .futureValue shouldBe Seq(1, 2)
     }
+    "Either flattenF" in {
+      Source(Seq(Right(Right(1)), Left(true), Right(Left(false)), Right(Right(2))))
+        .flattenF
+        .runWith(Sink.seq)
+        .futureValue shouldBe Seq(Right(1), Left(true), Left(false), Right(2))
+    }
     "Either flatMapMergeF" in {
       Source(Seq(Right(1), Left(true), Right(2)))
         .flatMapMergeF(8, element => Source(Seq(element * 2, element * 4)))
diff --git a/akka-stream-utils/shared/src/main/scala/StreamOps.scala b/akka-stream-utils/shared/src/main/scala/StreamOps.scala
index 1ec0d86..8e1d945 100644
--- a/akka-stream-utils/shared/src/main/scala/StreamOps.scala
+++ b/akka-stream-utils/shared/src/main/scala/StreamOps.scala
@@ -262,10 +262,16 @@ object StreamOps {
         case _ => Left(zero)
       })
 
-      def flattenF: s.Repr[OutR] = s.collect {
+      def collectRightF: s.Repr[OutR] = s.collect {
         case Right(value) => value
       }
 
+      def flattenF[OutR2](implicit ev: OutR <:< Either[OutL, OutR2]): s.Repr[Either[OutL, OutR2]] =
+        s.map {
+          case Left(outL) => Left(outL)
+          case Right(outR) => ev.apply(outR)
+        }
+
       def groupByF[K](maxSubstreams: Int, f: OutR => K): SubFlow[Either[OutL, OutR], Mat, s.Repr, s.Closed] = {
         s.groupBy(maxSubstreams, {
           case Right(value) => Some(f(value))