Oslar

const $inner = $('.inner');
if (!$inner.find('.butt_back').length) {
  $inner.append(...);
}

$('.inner')
  .find('.butt_back').remove()
  .end()
  .append(...);

<iframe width="560" height="315" src="https://www.youtube.com/embed/777tsCDqYo0?rel=0&modestbranding=1&autohide=1&mute=1&showinfo=0&controls=0&autoplay=1"  width="560" height="315" allowfullscreen></iframe>

<iframe width="560" height="315" src="https://www.youtube.com/embed/777tsCDqYo0?rel=0&modestbranding=1&autohide=1&mute=1&showinfo=0&controls=0&autoplay=1"  width="560" height="315" frameborder="0" allowfullscreen></iframe>

$('.list_add_room').on('click', '.butt_edit', function() {
  const $el = $(this).closest('.item_contain_list').find('p');
  $('.list_add_room [contenteditable="true"]').not($el).removeAttr('contenteditable');
  $el.attr('contenteditable', (i, val) => val !== 'true').focus();
});

$('.list_add_room').on('click', '.butt_edit', function() {
  $(this).closest('.item_contain_list').find('p').attr('contenteditable', 'true').focus();
}).on('blur', '[contenteditable="true"]', function() {
  $(this).removeAttr('contenteditable');
});

jQuery('.list_add_room').on('click', ' .butt_edit', function() {
  jQuery('.item_contain_list p').attr('contenteditable', false)
  jQuery(this).closest('.item_contain_list').find('p').attr('contenteditable', true).focus();
});

const $popup = $('#popup_idea');
const $list = $('.list_idea');

$list.on('click', '.open_win_idea', function() {
  $popup.data('del', $(this).closest('li').index()).fadeIn();
});

$('.close_win_butt_yes').click(function() {
  $list.children().eq($popup.data('del')).remove();
  $popup.fadeOut();
});

$('.close_win_butt_no, .close_win').click(function() {
  $popup.fadeOut();
});

//То что должны удалить
var target;

jQuery('.open_win_idea').click(function () {
  target = $(this).parent();
  jQuery('#popup_idea, .bg_body').fadeIn();
});

jQuery('.close_win_butt_no').click(function () {
  jQuery('#popup_idea, .bg_body').fadeOut();
});

jQuery('.close_win_butt_yes').click(function () {
 //Удаляем
  $(target).remove();
  jQuery('#popup_idea, .bg_body').fadeOut();
});

$('.list_idea').on('click', '.open_win_idea', function() {
  $(this)
    .closest('li')
    .find('p')
    .attr('contenteditable', (i, val) => val !== 'true')
    .focus();
});