diff --git a/saddle-core/src/main/scala/org/saddle/index/JoinerImpl.scala b/saddle-core/src/main/scala/org/saddle/index/JoinerImpl.scala
index 5a04d454..54335b58 100644
--- a/saddle-core/src/main/scala/org/saddle/index/JoinerImpl.scala
+++ b/saddle-core/src/main/scala/org/saddle/index/JoinerImpl.scala
@@ -28,8 +28,34 @@ class JoinerImpl[@spec(Boolean, Int, Long, Double) T: ST: ORD]
   private implicit def wrapArray(arr: Array[Int]): Option[Array[Int]] =
     Some(arr)
 
-  def join(left: Index[T], right: Index[T], how: JoinType): ReIndexer[T] = {
-    if (left == right) {
+  def join(left: Index[T], right: Index[T], how: JoinType): ReIndexer[T] =
+    join(left, right, how, false)
+
+  /** Perform database joins
+    *
+    * @param left
+    *   left index to join
+    * @param right
+    *   right index to join
+    * @param how
+    *   mode of operation: inner, left outer, right outer, full outer
+    * @param forceProperSemantics
+    *   if false, then no join is happening if left == right and right is
+    *   returned This is correct for unique indexes, and also practical
+    *   otherwise. If forceProperSemantics true, then the join is done even
+    *   between identical indexes. At the moment forceProperSemantics=true is
+    *   used at no places in saddle's source code (i.e. all frame joins etc use
+    *   the shortcut to not produce proper joins of identical indexes with
+    *   repeated values)
+    * @return
+    */
+  def join(
+      left: Index[T],
+      right: Index[T],
+      how: JoinType,
+      forceProperSemantics: Boolean
+  ): ReIndexer[T] = {
+    if (left == right && !forceProperSemantics) {
       ReIndexer(None, None, right)
     } else if (left.isUnique && right.isUnique) {
       how match {
diff --git a/saddle-core/src/test/scala/org/saddle/IndexSpec.scala b/saddle-core/src/test/scala/org/saddle/IndexSpec.scala
index 847e477a..66ce5639 100644
--- a/saddle-core/src/test/scala/org/saddle/IndexSpec.scala
+++ b/saddle-core/src/test/scala/org/saddle/IndexSpec.scala
@@ -12,9 +12,9 @@ import org.saddle.index.OuterJoin
 class IndexSpec extends Specification {
   "Index methods" should {
     "contiguous works" in {
-      val ix = Index(1,1,0,0)
+      val ix = Index(1, 1, 0, 0)
       ix.isContiguous must_== true
-    } 
+    }
     "over flow in join" in {
       val ix1 = Index(array.randInt(1000000, 0, 3))
       val ix2 = Index(array.randInt(10000, 0, 3))
@@ -42,7 +42,18 @@ class IndexSpec extends Specification {
   }
   "Index Joins" should {
 
-    "Outer join of same non-unique indexes " in {
+    "Outer join of same non-unique indexes with forceProperSemantics = true makes a join " in {
+      val ix1 = Index(0, 0)
+      val ix2 = Index(0, 0)
+      val res = (new org.saddle.index.JoinerImpl[Int])
+        .join(ix1, ix2, index.OuterJoin, forceProperSemantics = true)
+
+      res.index must_== Index(0, 0, 0, 0)
+      res.lTake.get must_== Array(0, 0, 1, 1)
+      res.rTake.get must_== Array(0, 1, 0, 1)
+
+    }
+    "Outer join of same non-unique indexes with forceProperSemantics=false (the default) returns the input " in {
       val ix1 = Index(0, 0)
       val ix2 = Index(0, 0)
       val res = ix1.join(ix2, how = index.OuterJoin)