fix multiple handler notifications (#19655)

Fixes #19647
Adds integration test to catch multiple handler notifications

test/integration/targets/handlers/roles/test_handlers_meta/handlers/main.yml

@@ -5,3 +5,6 @@
 - name: set_handler_fact_2
   set_fact:
     handler2_called: True
+
+- name: count_handler
+  shell: echo . >> {{ handler_countpath }}

test/integration/targets/handlers/roles/test_handlers_meta/tasks/main.yml

@@ -30,6 +30,41 @@
         - "handler1_called is defined"
         - "handler2_called is not defined"
 
+- name: make a tempfile for counting
+  shell: mktemp
+  register: mktemp_out
+
+- name: register tempfile path
+  set_fact:
+    handler_countpath: "{{ mktemp_out.stdout }}"
+
+- name: notify the counting handler
+  shell: echo
+  notify:
+    - count_handler
+
+- name: notify the counting handler again
+  shell: echo
+  notify:
+    - count_handler
+
+- name: force handler execution now
+  meta: flush_handlers
+
+- name: get handler execution count
+  shell: cat {{ handler_countpath }} | grep -o . | wc -l
+  register: exec_count_out
+
+- debug: var=exec_count_out.stdout
+
+- name: ensure single execution
+  assert:
+    that:
+    - exec_count_out.stdout | int == 1
+
+- name: cleanup tempfile
+  file: path={{ handler_countpath }} state=absent
+
 - name: reset handler1_called
   set_fact:
     handler1_called: False