diff --git a/core/task/manager.go b/core/task/manager.go
index 587f6f7f..bbcd067f 100644
--- a/core/task/manager.go
+++ b/core/task/manager.go
@@ -100,6 +100,7 @@ type Manager struct {
 	schedulerState  *schedulerState
 	internalEventCh chan<- event.Event
 	ackKilledTasks  *safeAcks
+	killTasksMu     sync.Mutex // to avoid races when attempting to kill the same tasks in different goroutines
 }
 
 func NewManager(shutdown func(), internalEventCh chan<- event.Event) (taskman *Manager, err error) {
@@ -1042,7 +1043,7 @@ func (m *Manager) Cleanup() (killed Tasks, running Tasks, err error) {
 // If the task list includes locked tasks, TaskNotFoundError is returned.
 func (m *Manager) KillTasks(taskIds []string) (killed Tasks, running Tasks, err error) {
 	taskCanBeKilledFilter := func(t *Task) bool {
-		if t.IsLocked() {
+		if t.IsLocked() || m.ackKilledTasks.contains(t.taskId) {
 			return false
 		}
 		for _, id := range taskIds {
@@ -1053,13 +1054,18 @@ func (m *Manager) KillTasks(taskIds []string) (killed Tasks, running Tasks, err
 		return false
 	}
 
+	if !m.killTasksMu.TryLock() {
+		log.WithField("level", infologger.IL_Support).Warnf("Scheduling killing tasks was delayed until another goroutine is done doing so")
+		m.killTasksMu.Lock()
+		log.WithField("level", infologger.IL_Support).Infof("Scheduling killing tasks is resumed")
+	}
 	// TODO: use grouping instead of 2 passes of filtering for performance
 	toKill := m.roster.filtered(taskCanBeKilledFilter)
-	unkillable := m.roster.filtered(func(t *Task) bool { return !taskCanBeKilledFilter(t) })
 
 	if len(toKill) < len(taskIds) {
+		unkillable := m.roster.filtered(func(t *Task) bool { return !taskCanBeKilledFilter(t) })
 		log.WithField("taskIds", strings.Join(unkillable.GetTaskIds(), ", ")).
-			Debugf("some tasks cannot be physically killed (already dead?), will instead only be removed from roster")
+			Debugf("some tasks cannot be physically killed (already dead or being killed in another goroutine?), will instead only be removed from roster")
 	}
 
 	for _, id := range toKill.GetTaskIds() {
@@ -1067,6 +1073,8 @@ func (m *Manager) KillTasks(taskIds []string) (killed Tasks, running Tasks, err
 	}
 
 	killed, running, err = m.doKillTasks(toKill)
+	m.killTasksMu.Unlock()
+
 	for _, id := range killed.GetTaskIds() {
 		ack, ok := m.ackKilledTasks.getValue(id)
 		if ok {
@@ -1088,6 +1096,7 @@ func (m *Manager) doKillTasks(tasks Tasks) (killed Tasks, running Tasks, err err
 	inactiveTasks := tasks.Filtered(func(task *Task) bool {
 		return task.status != ACTIVE
 	})
+
 	// Remove from the roster the tasks which are also in the inactiveTasks list to delete
 	m.roster.updateTasks(m.roster.filtered(func(task *Task) bool {
 		return !inactiveTasks.Contains(func(t *Task) bool {