diff --git a/src/main/java/com/ffii/fpsms/modules/deliveryOrder/service/DoWorkbenchReleaseService.kt b/src/main/java/com/ffii/fpsms/modules/deliveryOrder/service/DoWorkbenchReleaseService.kt
index c45e8a8..f349b2c 100644
--- a/src/main/java/com/ffii/fpsms/modules/deliveryOrder/service/DoWorkbenchReleaseService.kt
+++ b/src/main/java/com/ffii/fpsms/modules/deliveryOrder/service/DoWorkbenchReleaseService.kt
@@ -1066,24 +1066,31 @@ open class DoWorkbenchReleaseService(
         )
     }
 
+    private fun isValidMergeLeftTicket(row: DopoMergeRow): Boolean {
+        val rt = row.releaseType?.trim()?.lowercase().orEmpty()
+        if (isWorkbenchMergeTicketNo(row.ticketNo)) {
+            return rt == WorkbenchReleaseTypeSupport.IS_EXTRA_BATCH.lowercase() ||
+                rt == WorkbenchReleaseTypeSupport.IS_EXTRA_SINGLE.lowercase()
+        }
+        return rt == WorkbenchReleaseTypeSupport.BATCH.lowercase() ||
+            rt == WorkbenchReleaseTypeSupport.SINGLE.lowercase()
+    }
+
     @Transactional(rollbackFor = [Exception::class])
     open fun mergeTicketsCase3(batchOrSingleDopoId: Long, isExtraDopoId: Long): MessageResponse {
         if (batchOrSingleDopoId == isExtraDopoId) {
             return mergeTicketsError("SAME_TICKET", "Cannot merge a ticket with itself")
         }
         val batchRow = loadDopoRowForMerge(batchOrSingleDopoId)
-            ?: return mergeTicketsError("NOT_FOUND", "Batch/single ticket not found")
+            ?: return mergeTicketsError("NOT_FOUND", "Left ticket not found")
         val extraRow = loadDopoRowForMerge(isExtraDopoId)
             ?: return mergeTicketsError("NOT_FOUND", "isExtra ticket not found")
 
-        val batchRt = batchRow.releaseType?.trim()?.lowercase().orEmpty()
-        if (batchRt != WorkbenchReleaseTypeSupport.BATCH.lowercase() &&
-            batchRt != WorkbenchReleaseTypeSupport.SINGLE.lowercase()
-        ) {
-            return mergeTicketsError("INVALID_BATCH", "Left ticket must be plain batch or single")
-        }
-        if (isWorkbenchMergeTicketNo(batchRow.ticketNo)) {
-            return mergeTicketsError("INVALID_BATCH", "Left ticket must not be a TI-M merge ticket")
+        if (!isValidMergeLeftTicket(batchRow)) {
+            return mergeTicketsError(
+                "INVALID_BATCH",
+                "Left ticket must be plain batch/single or an active TI-M merge ticket",
+            )
         }
         if (extraRow.releaseType?.trim()?.lowercase() != WorkbenchReleaseTypeSupport.LEGACY_IS_EXTRA.lowercase()) {
             return mergeTicketsError("INVALID_EXTRA", "Right ticket must be releaseType isExtra")
@@ -1103,23 +1110,35 @@ open class DoWorkbenchReleaseService(
             )
         }
 
-        val isSingleRelease = batchRt == WorkbenchReleaseTypeSupport.SINGLE.lowercase()
-        val requiredDate = batchRow.requiredDeliveryDate ?: LocalDate.now()
-        val storeId = batchRow.storeId
-        val isDefaultTruckLane = storeId.isNullOrBlank()
-        val ticketFloorSegment = resolveTicketFloorSegment(storeId, isDefaultTruckLane)
+        val leftIsExistingMerge = isWorkbenchMergeTicketNo(batchRow.ticketNo)
+        val batchRt = batchRow.releaseType?.trim()?.lowercase().orEmpty()
+        val isSingleRelease = when {
+            leftIsExistingMerge ->
+                batchRt == WorkbenchReleaseTypeSupport.IS_EXTRA_SINGLE.lowercase()
+            else -> batchRt == WorkbenchReleaseTypeSupport.SINGLE.lowercase()
+        }
 
-        val mergeHeaderId = insertMergeHeaderFromDopoRow(
-            template = batchRow,
-            isSingleRelease = isSingleRelease,
-            requiredDate = requiredDate,
-            ticketFloorSegment = ticketFloorSegment,
-        ) ?: return mergeTicketsError("CREATE_FAILED", "Failed to create TI-M merge ticket")
+        val mergeHeaderId = if (leftIsExistingMerge) {
+            batchOrSingleDopoId
+        } else {
+            val requiredDate = batchRow.requiredDeliveryDate ?: LocalDate.now()
+            val storeId = batchRow.storeId
+            val isDefaultTruckLane = storeId.isNullOrBlank()
+            val ticketFloorSegment = resolveTicketFloorSegment(storeId, isDefaultTruckLane)
+            insertMergeHeaderFromDopoRow(
+                template = batchRow,
+                isSingleRelease = isSingleRelease,
+                requiredDate = requiredDate,
+                ticketFloorSegment = ticketFloorSegment,
+            ) ?: return mergeTicketsError("CREATE_FAILED", "Failed to create TI-M merge ticket")
+        }
 
-        retireSourceTicketIntoMergeHeader(batchOrSingleDopoId, mergeHeaderId)
+        if (!leftIsExistingMerge) {
+            retireSourceTicketIntoMergeHeader(batchOrSingleDopoId, mergeHeaderId)
+        }
         retireSourceTicketIntoMergeHeader(isExtraDopoId, mergeHeaderId)
 
-        val newTicketNo = jdbcDao.queryForList(
+        val mergeTicketNo = jdbcDao.queryForList(
             "SELECT ticketNo AS t FROM fpsmsdb.delivery_order_pick_order WHERE id = :id",
             mapOf("id" to mergeHeaderId),
         ).firstOrNull()?.let { row ->
@@ -1131,13 +1150,14 @@ open class DoWorkbenchReleaseService(
             code = "SUCCESS",
             name = "workbench_merge_tickets",
             type = "workbench_merge_tickets",
-            message = "Merged into $newTicketNo",
+            message = "Merged into $mergeTicketNo",
             errorPosition = null,
             entity = mapOf(
                 "mergeDeliveryOrderPickOrderId" to mergeHeaderId,
-                "ticketNo" to newTicketNo,
+                "ticketNo" to mergeTicketNo,
                 "batchOrSingleDopoId" to batchOrSingleDopoId,
                 "isExtraDopoId" to isExtraDopoId,
+                "mergedIntoExistingTiM" to leftIsExistingMerge,
             ),
         )
     }
@@ -1205,8 +1225,10 @@ open class DoWorkbenchReleaseService(
         when (side) {
             "batchFamily" -> sql.append(
                 """
-                 AND LOWER(COALESCE(dop.releaseType, '')) IN ('batch', 'single')
-                 AND (dop.ticketNo IS NULL OR dop.ticketNo NOT LIKE 'TI-M-%')
+                 AND (
+                   LOWER(COALESCE(dop.releaseType, '')) IN ('batch', 'single')
+                   OR dop.ticketNo LIKE 'TI-M-%'
+                 )
                 """.trimIndent().let { " $it" },
             )
             "isExtra" -> sql.append(" AND LOWER(COALESCE(dop.releaseType, '')) = 'isextra' ")
diff --git a/src/main/java/com/ffii/fpsms/modules/deliveryOrder/web/DoWorkbenchController.kt b/src/main/java/com/ffii/fpsms/modules/deliveryOrder/web/DoWorkbenchController.kt
index 1761002..702db0a 100644
--- a/src/main/java/com/ffii/fpsms/modules/deliveryOrder/web/DoWorkbenchController.kt
+++ b/src/main/java/com/ffii/fpsms/modules/deliveryOrder/web/DoWorkbenchController.kt
@@ -227,7 +227,7 @@ class DoWorkbenchController(
     ): WorkbenchMergeTicketCandidatesResponse =
         doWorkbenchReleaseService.getMergeTicketCandidates(requiredDate, shopSearch)
 
-    /** Case 3: merge selected batch/single + isExtra into a new [TI-M] ticket. */
+    /** Case 3 / 3b: merge batch/single or existing [TI-M] + isExtra into [TI-M]. */
     @PostMapping("/merge-tickets")
     fun mergeWorkbenchTickets(@RequestBody request: WorkbenchMergeTicketsRequest): MessageResponse =
         doWorkbenchReleaseService.mergeTicketsCase3(