diff --git a/backend/src/main/kotlin/org/tormap/service/CoalesceService.kt b/backend/src/main/kotlin/org/tormap/service/CoalesceService.kt
index 6a0bfda4..aee22173 100644
--- a/backend/src/main/kotlin/org/tormap/service/CoalesceService.kt
+++ b/backend/src/main/kotlin/org/tormap/service/CoalesceService.kt
@@ -32,21 +32,33 @@ class CoalesceService(
     private val slots = ConcurrentHashMap<String, Slot>()
 
     fun submitAsync(key: String, task: () -> Unit): CompletableFuture<Void> {
-        val slot = slots.computeIfAbsent(key) { Slot() }
+        while (true) {
+            val slot = slots.computeIfAbsent(key) { Slot() }
+            var firstFuture: CompletableFuture<Void>? = null
 
-        // If no run is in flight, start the loop and return a future for the first run.
-        if (slot.running.compareAndSet(false, true)) {
-            val firstFuture = CompletableFuture<Void>()
-            runLoopAsync(key, slot, task, firstFuture)
-            return firstFuture
-        }
+            val queuedFuture = synchronized(slot.monitor) {
+                if (slots[key] !== slot) return@synchronized null
+
+                // If no run is in flight, start the loop and return a future for the first run.
+                if (!slot.running.get()) {
+                    firstFuture = CompletableFuture<Void>()
+                    slot.running.set(true)
+                    return@synchronized null
+                }
+
+                // Already running: request one rerun (collapsed) and return the shared future for that rerun.
+                slot.rerunRequested.set(true)
+                val existing = slot.nextFuture
+                if (existing != null) return@synchronized existing
+                CompletableFuture<Void>().also { slot.nextFuture = it }
+            }
 
-        // Already running: request one rerun (collapsed) and return the shared future for that rerun.
-        slot.rerunRequested.set(true)
-        synchronized(slot.monitor) {
-            val existing = slot.nextFuture
-            if (existing != null) return existing
-            return CompletableFuture<Void>().also { slot.nextFuture = it }
+            if (queuedFuture != null) return queuedFuture
+            val startFuture = firstFuture
+            if (startFuture != null) {
+                runLoopAsync(key, slot, task, startFuture)
+                return startFuture
+            }
         }
     }
 
@@ -70,28 +82,32 @@ class CoalesceService(
                     } catch (ex: Exception) {
                         logger.error("Coalesced task failed for key={}", key, ex)
                         currentFuture?.completeExceptionally(ex)
+                        synchronized(slot.monitor) {
+                            val pendingNext = slot.nextFuture
+                            slot.nextFuture = null
+                            slot.rerunRequested.set(false)
+                            slot.running.set(false)
+                            slots.remove(key, slot)
+                            pendingNext?.completeExceptionally(ex)
+                        }
                         // Keep existing behavior: stop reruns for this cycle after a failed execution.
                         keepGoing = false
-                        continue
                     }
 
-                    // If no rerun requested, finish.
-                    if (!slot.rerunRequested.getAndSet(false)) {
-                        keepGoing = false
-                        continue
-                    }
-
-                    // Promote the shared next future for the rerun. If none exists (rare race), create one.
-                    currentFuture = synchronized(slot.monitor) {
-                        val next = slot.nextFuture ?: CompletableFuture<Void>()
-                        slot.nextFuture = null
-                        next
+                    synchronized(slot.monitor) {
+                        // If no rerun requested, finish.
+                        if (!slot.rerunRequested.getAndSet(false)) {
+                            slot.running.set(false)
+                            slots.remove(key, slot)
+                            keepGoing = false
+                        } else {
+                            // Promote the shared next future for the rerun. If none exists (rare race), create one.
+                            val next = slot.nextFuture ?: CompletableFuture<Void>()
+                            slot.nextFuture = null
+                            currentFuture = next
+                        }
                     }
                 }
-
-                // Mark not running and remove slot. If a submit races here, it will start a new loop.
-                slot.running.set(false)
-                slots.remove(key, slot)
             },
             coalesceExecutor,
         )