diff --git a/src/action.c b/src/action.c
index de8c3374..1362dff6 100644
--- a/src/action.c
+++ b/src/action.c
@@ -14,14 +14,18 @@ void Action_prepare(Trigger *self, Event *event) {
   LF_DEBUG(TRIG, "Preparing action %p", self);
   Action *act = (Action *)self;
   Scheduler *sched = &self->parent->env->scheduler;
-  self->is_present = true;
   memcpy(act->value_ptr, event->payload, act->payload_pool.size);
 
-  sched->register_for_cleanup(sched, self);
-
-  for (size_t i = 0; i < act->effects.size; i++) {
-    validaten(sched->reaction_queue.insert(&sched->reaction_queue, act->effects.reactions[i]));
+  if (self->is_present) {
+    LF_WARN(TRIG, "Action %p is already present at this tag. Its value was overwritten", self);
+  } else {
+    sched->register_for_cleanup(sched, self);
+    for (size_t i = 0; i < act->effects.size; i++) {
+      validate(sched->reaction_queue.insert(&sched->reaction_queue, act->effects.reactions[i]) == LF_OK);
+    }
   }
+
+  self->is_present = true;
   self->payload_pool->free(self->payload_pool, event->payload);
 }
 
diff --git a/src/queues.c b/src/queues.c
index 8b2daee1..09fffce5 100644
--- a/src/queues.c
+++ b/src/queues.c
@@ -86,6 +86,11 @@ lf_ret_t ReactionQueue_insert(ReactionQueue *self, Reaction *reaction) {
   validate(self->level_size[reaction->level] < REACTION_QUEUE_SIZE);
   validate(self->curr_level <= reaction->level);
 
+  for (int i = 0; i < self->level_size[reaction->level]; i++) {
+    if (self->array[reaction->level][i] == reaction) {
+      return LF_OK;
+    }
+  }
   self->array[reaction->level][self->level_size[reaction->level]++] = reaction;
   if (reaction->level > self->max_active_level) {
     self->max_active_level = reaction->level;
diff --git a/test/unit/action_overwrite_test.c b/test/unit/action_overwrite_test.c
new file mode 100644
index 00000000..070305d7
--- /dev/null
+++ b/test/unit/action_overwrite_test.c
@@ -0,0 +1,68 @@
+#include "reactor-uc/reactor-uc.h"
+#include "unity.h"
+
+DEFINE_ACTION_STRUCT(MyAction, LOGICAL_ACTION, 1, 1, int, 2);
+DEFINE_ACTION_CTOR_FIXED(MyAction, LOGICAL_ACTION, 1, 1, int, 2, MSEC(0));
+DEFINE_STARTUP_STRUCT(MyStartup, 1);
+DEFINE_STARTUP_CTOR(MyStartup, 1)
+DEFINE_REACTION_STRUCT(MyReactor, 0, 1);
+
+typedef struct {
+  Reactor super;
+  MyReactor_Reaction0 my_reaction;
+  MyAction my_action;
+  MyStartup startup;
+  Reaction *_reactions[1];
+  Trigger *_triggers[2];
+  int cnt;
+} MyReactor;
+
+DEFINE_REACTION_BODY(MyReactor, 0) {
+  MyReactor *self = (MyReactor *)_self->parent;
+  MyAction *my_action = &self->my_action;
+  if (self->cnt == 0) {
+    TEST_ASSERT_EQUAL(lf_is_present(my_action), false);
+    lf_schedule(my_action, 41, MSEC(1));
+    lf_schedule(my_action, 42, MSEC(1));
+  } else {
+    TEST_ASSERT_EQUAL(1, self->cnt);
+    TEST_ASSERT_EQUAL(lf_is_present(my_action), true);
+    TEST_ASSERT_EQUAL(42, my_action->value);
+  }
+  self->cnt++;
+}
+
+DEFINE_REACTION_CTOR(MyReactor, 0);
+
+void MyReactor_ctor(MyReactor *self, Environment *env) {
+  self->_reactions[0] = (Reaction *)&self->my_reaction;
+  self->_triggers[0] = (Trigger *)&self->startup;
+  self->_triggers[1] = (Trigger *)&self->my_action;
+  Reactor_ctor(&self->super, "MyReactor", env, NULL, NULL, 0, self->_reactions, 1, self->_triggers, 2);
+  MyAction_ctor(&self->my_action, &self->super);
+  MyReactor_Reaction0_ctor(&self->my_reaction, &self->super);
+  MyStartup_ctor(&self->startup, &self->super);
+  ACTION_REGISTER_EFFECT(self->my_action, self->my_reaction);
+  REACTION_REGISTER_EFFECT(self->my_reaction, self->my_action);
+  ACTION_REGISTER_SOURCE(self->my_action, self->my_reaction);
+  BUILTIN_REGISTER_EFFECT(self->startup, self->my_reaction);
+
+  self->cnt = 0;
+}
+
+void test_simple() {
+  MyReactor my_reactor;
+  Environment env;
+  Environment_ctor(&env, (Reactor *)&my_reactor);
+  MyReactor_ctor(&my_reactor, &env);
+  env.scheduler.duration = MSEC(100);
+  env.assemble(&env);
+  env.start(&env);
+  Environment_free(&env);
+}
+
+int main() {
+  UNITY_BEGIN();
+  RUN_TEST(test_simple);
+  return UNITY_END();
+}
\ No newline at end of file
diff --git a/test/unit/multiple_startup_shutdown_test.c b/test/unit/multiple_startup_shutdown_test.c
new file mode 100644
index 00000000..caeeb25a
--- /dev/null
+++ b/test/unit/multiple_startup_shutdown_test.c
@@ -0,0 +1,96 @@
+
+#include "reactor-uc/reactor-uc.h"
+#include "unity.h"
+
+DEFINE_STARTUP_STRUCT(MyStartup, 2)
+DEFINE_STARTUP_CTOR(MyStartup, 1)
+
+DEFINE_STARTUP_STRUCT(MyStartup2, 1)
+DEFINE_STARTUP_CTOR(MyStartup2, 1)
+
+DEFINE_SHUTDOWN_STRUCT(MyShutdown, 1)
+DEFINE_SHUTDOWN_CTOR(MyShutdown, 1)
+
+DEFINE_SHUTDOWN_STRUCT(MyShutdown2, 1)
+DEFINE_SHUTDOWN_CTOR(MyShutdown2, 1)
+
+DEFINE_REACTION_STRUCT(MyReactor, 0, 0)
+DEFINE_REACTION_STRUCT(MyReactor, 1, 0)
+DEFINE_REACTION_STRUCT(MyReactor, 2, 0)
+DEFINE_REACTION_STRUCT(MyReactor, 3, 0)
+
+typedef struct {
+  Reactor super;
+  MyReactor_Reaction0 reaction0;
+  MyReactor_Reaction1 reaction1;
+  MyReactor_Reaction2 reaction2;
+  MyReactor_Reaction3 reaction3;
+  MyStartup startup;
+  MyStartup2 startup2;
+  MyShutdown shutdown;
+  MyShutdown2 shutdown2;
+  Reaction *_reactions[4];
+  Trigger *_triggers[4];
+  int cnt;
+} MyReactor;
+
+DEFINE_REACTION_BODY(MyReactor, 0) {
+  MyReactor *self = (MyReactor *)_self->parent;
+  TEST_ASSERT_EQUAL(0, self->cnt++);
+}
+
+DEFINE_REACTION_BODY(MyReactor, 1) {
+  MyReactor *self = (MyReactor *)_self->parent;
+  TEST_ASSERT_EQUAL(1, self->cnt++);
+}
+
+DEFINE_REACTION_BODY(MyReactor, 2) {
+  MyReactor *self = (MyReactor *)_self->parent;
+  TEST_ASSERT_EQUAL(2, self->cnt++);
+}
+
+DEFINE_REACTION_BODY(MyReactor, 3) {
+  MyReactor *self = (MyReactor *)_self->parent;
+  TEST_ASSERT_EQUAL(3, self->cnt++);
+}
+
+DEFINE_REACTION_CTOR(MyReactor, 0)
+DEFINE_REACTION_CTOR(MyReactor, 1)
+DEFINE_REACTION_CTOR(MyReactor, 2)
+DEFINE_REACTION_CTOR(MyReactor, 3)
+
+void MyReactor_ctor(MyReactor *self, Environment *env) {
+  self->_reactions[0] = (Reaction *)&self->reaction0;
+  self->_reactions[1] = (Reaction *)&self->reaction1;
+  self->_reactions[2] = (Reaction *)&self->reaction2;
+  self->_reactions[3] = (Reaction *)&self->reaction3;
+  self->_triggers[0] = (Trigger *)&self->startup;
+  self->_triggers[1] = (Trigger *)&self->shutdown;
+  self->_triggers[2] = (Trigger *)&self->startup2;
+  self->_triggers[3] = (Trigger *)&self->shutdown2;
+
+  Reactor_ctor(&self->super, "MyReactor", env, NULL, NULL, 0, self->_reactions, 4, self->_triggers, 4);
+  MyReactor_Reaction0_ctor(&self->reaction0, &self->super);
+  MyReactor_Reaction1_ctor(&self->reaction1, &self->super);
+  MyReactor_Reaction2_ctor(&self->reaction2, &self->super);
+  MyReactor_Reaction3_ctor(&self->reaction3, &self->super);
+
+  MyStartup_ctor(&self->startup, &self->super);
+  MyShutdown_ctor(&self->shutdown, &self->super);
+  MyStartup2_ctor(&self->startup2, &self->super);
+  MyShutdown2_ctor(&self->shutdown2, &self->super);
+
+  BUILTIN_REGISTER_EFFECT(self->startup, self->reaction0);
+  BUILTIN_REGISTER_EFFECT(self->startup, self->reaction1);
+  BUILTIN_REGISTER_EFFECT(self->startup2, self->reaction1);
+  BUILTIN_REGISTER_EFFECT(self->shutdown, self->reaction2);
+  BUILTIN_REGISTER_EFFECT(self->shutdown2, self->reaction3);
+}
+
+ENTRY_POINT(MyReactor, FOREVER, false)
+
+int main() {
+  UNITY_BEGIN();
+  RUN_TEST(lf_start);
+  return UNITY_END();
+}