"Modified twice"

components.my-child-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNames: ['my-child-component']
});

components.my-component.js

import Ember from 'ember';
import run from 'ember-runloop';

export default Ember.Component.extend({
  classNames: ['my-component'],
  classNameBindings: ['isFocused'],
  attributeBindings: ['tabindex'],
  
  tabindex: '0',
  
  focusIn() {
    this._super(...arguments);
    this.set('isFocused', true); 
  },
  
  focusOut() {
    this._super(...arguments);
    this.set('isFocused', false);
  },
  
  actions: {
    open() {
    	this.set('isOpen', true);  
    },
    
    close() {
      this.set('isOpen', false);
    }
  }
});

styles.app.css

.my-component {
  border: 2px solid red;
  min-height: 1em;
  padding: 1em;
  outline: none;
}

.my-child-component {
  border: 2px solid blue;
  min-height: 1em;
  padding: 1em;
}

.my-component.is-focused {
  border: 2px solid green;
}

templates.application.hbs

{{#my-component as |mc|}}
  <button onclick={{action mc.open}}>open</button>    
  <button onclick={{action mc.close}}>close</button>

  <br><br>

  {{#mc.child}}
    <button onclick={{action mc.close}}>
      close (causes double render error)
    </button>
  {{/mc.child}}
{{/my-component}}

templates.components.my-child-component.hbs

{{yield}}

templates.components.my-component.hbs

Focused?: {{this.isFocused}}

<br><br>

{{yield (hash
  open=(action 'open')
  close=(action 'close')
  child=(if this.isOpen (component 'my-child-component'))
)}}

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}