diff --git a/openrtb_ext/convert_up.go b/openrtb_ext/convert_up.go
index cfc9259b20..e918e9bfbe 100644
--- a/openrtb_ext/convert_up.go
+++ b/openrtb_ext/convert_up.go
@@ -147,17 +147,50 @@ func moveUSPrivacyFrom25To26(r *RequestWrapper) {
 
 // moveEIDFrom25To26 modifies the request to move the OpenRTB 2.5 external identifiers
 // (req.user.ext.eids) to the OpenRTB 2.6 location (req.user.eids). If the OpenRTB 2.6
-// location is already present the OpenRTB 2.5 external identifiers is dropped.
+// location is already present the OpenRTB 2.5 external identifiers are appended for
+// sources not already in the eids array
 func moveEIDFrom25To26(r *RequestWrapper) {
-	// read and clear 2.5 location
+	// Read 2.5 location
 	userExt, _ := r.GetUserExt()
-	eid25 := userExt.GetEid()
+	eids25 := userExt.GetEid()
+
+	// Clear 2.5 location
 	userExt.SetEid(nil)
 
-	// move to 2.6 location
-	if eid25 != nil && r.User.EIDs == nil {
-		r.User.EIDs = *eid25
+	// If there's no eids25, there's nothing to move or merge
+	if eids25 == nil || len(*eids25) == 0 {
+		return
+	}
+
+	// If there's no 2.6 location, simply set eids
+	if r.User.EIDs == nil {
+		r.User.EIDs = *eids25
+		return
+	}
+
+	// If 2.6 location is present, append only new eids based on 'source'.
+	existingEIDs := r.User.EIDs
+	for _, eid25 := range *eids25 {
+		// If source is empty, append
+		if eid25.Source == "" {
+			existingEIDs = append(existingEIDs, eid25)
+			continue
+		}
+
+		found := false
+		for _, eid26 := range existingEIDs {
+			// Compare strings directly
+			if eid25.Source == eid26.Source {
+				found = true
+				break
+			}
+		}
+
+		if !found {
+			existingEIDs = append(existingEIDs, eid25)
+		}
 	}
+	r.User.EIDs = existingEIDs
 }
 
 // moveRewardedFromPrebidExtTo26 modifies the impression to move the Prebid specific
diff --git a/openrtb_ext/convert_up_test.go b/openrtb_ext/convert_up_test.go
index 2b5c230389..eaf22e536c 100644
--- a/openrtb_ext/convert_up_test.go
+++ b/openrtb_ext/convert_up_test.go
@@ -374,9 +374,32 @@ func TestMoveEIDFrom25To26(t *testing.T) {
 			expectedRequest: openrtb2.BidRequest{User: &openrtb2.User{EIDs: eid1}},
 		},
 		{
-			description:     "2.5 Dropped",
-			givenRequest:    openrtb2.BidRequest{User: &openrtb2.User{EIDs: eid1, Ext: eid2Json}},
-			expectedRequest: openrtb2.BidRequest{User: &openrtb2.User{EIDs: eid1}},
+			description: "2.5 Appended Because Different Source from all 2.6 eids",
+			givenRequest: openrtb2.BidRequest{
+				User: &openrtb2.User{
+					EIDs: []openrtb2.EID{{Source: "1"}},
+					Ext:  eid2Json,
+				},
+			},
+			expectedRequest: openrtb2.BidRequest{
+				User: &openrtb2.User{
+					EIDs: []openrtb2.EID{{Source: "1"}, {Source: "2"}},
+				},
+			},
+		},
+		{
+			description: "2.5 Not Appended Because Same Source exists IN 2.6",
+			givenRequest: openrtb2.BidRequest{
+				User: &openrtb2.User{
+					EIDs: []openrtb2.EID{{Source: "1"}},
+					Ext:  eid1Json,
+				},
+			},
+			expectedRequest: openrtb2.BidRequest{
+				User: &openrtb2.User{
+					EIDs: []openrtb2.EID{{Source: "1"}},
+				},
+			},
 		},
 		{
 			description:     "2.6 Left Alone",