interdiff impossible; taking evasive action
reverted:
--- b/src/Plugin/Field/FieldWidget/InlineParagraphsWidget.php
+++ a/src/Plugin/Field/FieldWidget/InlineParagraphsWidget.php
@@ -391,30 +391,11 @@
             '#paragraphs_mode' => 'edit',
           );
 
-          $links['duplicate_button'] = array(
-            '#type' => 'submit',
-            '#value' => $this->t('Duplicate'),
-            '#name' => strtr($id_prefix, '-', '_') . '_duplicate',
-            '#weight' => 502,
-            '#submit' => array(array(get_class($this), 'duplicateSubmit')),
-            '#limit_validation_errors' => array(array_merge($parents, array($field_name, 'add_more'))),
-            '#delta' => $delta,
-            '#ajax' => array(
-              'callback' => array(get_class($this), 'addMoreAjax'),
-              'wrapper' => $widget_state['ajax_wrapper_id'],
-              'effect' => 'fade',
-            ),
-            '#access' => $paragraphs_entity->access('update'),
-            '#prefix' => '<li class="duplicate">',
-            '#suffix' => '</li>',
-            '#paragraphs_mode' => 'duplicate',
-          );
-
           $links['remove_button'] = array(
             '#type' => 'submit',
             '#value' => $this->t('Remove'),
             '#name' => strtr($id_prefix, '-', '_') . '_remove',
+            '#weight' => 502,
-            '#weight' => 503,
             '#submit' => array(array(get_class($this), 'paragraphsItemSubmit')),
             '#limit_validation_errors' => array(array_merge($parents, array($field_name, 'add_more'))),
             '#delta' => $delta,
@@ -997,25 +978,6 @@
     $form_state->setRebuild();
   }
 
-  public static function duplicateSubmit(array $form, FormStateInterface $form_state) {
-    $button = $form_state->getTriggeringElement();
-    // Go one level up in the form, to the widgets container.
-    $element = NestedArray::getValue($form, array_slice($button['#array_parents'], 0, -4));
-    $field_name = $element['#field_name'];
-    $parents = $element['#field_parents'];
-
-    // Inserting new element in the array.
-    $field_state = static::getWidgetState($parents, $field_name, $form_state);
-    $delta = $button['#delta'];
-    array_splice($field_state['paragraphs'], $delta + 1, 0, $field_state['paragraphs'][$delta]);
-    $field_state['items_count']++;
-    $field_state['paragraphs'][$delta + 1]['entity'] = $field_state['paragraphs'][$delta]['entity']->createDuplicate();
-
-    static::setWidgetState($parents, $field_name, $form_state, $field_state);
-
-    $form_state->setRebuild();
-  }
-
   public static function paragraphsItemSubmit(array $form, FormStateInterface $form_state) {
     $button = $form_state->getTriggeringElement();
 
unchanged:
--- a/src/Plugin/Field/FieldWidget/InlineParagraphsWidget.php
+++ b/src/Plugin/Field/FieldWidget/InlineParagraphsWidget.php
@@ -391,11 +391,30 @@ class InlineParagraphsWidget extends WidgetBase {
             '#paragraphs_mode' => 'edit',
           );
 
+          $links['duplicate_button'] = array(
+            '#type' => 'submit',
+            '#value' => $this->t('Duplicate'),
+            '#name' => strtr($id_prefix, '-', '_') . '_duplicate',
+            '#weight' => 502,
+            '#submit' => array(array(get_class($this), 'duplicateSubmit')),
+            '#limit_validation_errors' => array(array_merge($parents, array($field_name, 'add_more'))),
+            '#delta' => $delta,
+            '#ajax' => array(
+              'callback' => array(get_class($this), 'itemAjax'),
+              'wrapper' => $widget_state['ajax_wrapper_id'],
+              'effect' => 'fade',
+            ),
+            '#access' => $paragraphs_entity->access('update'),
+            '#prefix' => '<li class="duplicate">',
+            '#suffix' => '</li>',
+            '#paragraphs_mode' => 'duplicate',
+          );
+
           $links['remove_button'] = array(
             '#type' => 'submit',
             '#value' => $this->t('Remove'),
             '#name' => strtr($id_prefix, '-', '_') . '_remove',
-            '#weight' => 502,
+            '#weight' => 503,
             '#submit' => array(array(get_class($this), 'paragraphsItemSubmit')),
             '#limit_validation_errors' => array(array_merge($parents, array($field_name, 'add_more'))),
             '#delta' => $delta,
@@ -963,6 +982,30 @@ class InlineParagraphsWidget extends WidgetBase {
     $form_state->setRebuild();
   }
 
+  public static function duplicateSubmit(array $form, FormStateInterface $form_state) {
+    $button = $form_state->getTriggeringElement();
+    // Go one level up in the form, to the widgets container.
+    $element = NestedArray::getValue($form, array_slice($button['#array_parents'], 0, -4));
+    $field_name = $element['#field_name'];
+    $parents = $element['#field_parents'];
+
+    // Inserting new element in the array.
+    $field_state = static::getWidgetState($parents, $field_name, $form_state);
+    $delta = $button['#delta'];
+    $field_state['items_count']++;
+    $field_state['real_item_count']++;
+    $field_state['original_deltas'] = array_merge($field_state['original_deltas'], ['1' => 1]) ;
+
+    $field_state['paragraphs'][] = [
+      'entity' => $field_state['paragraphs'][$delta]['entity']->createDuplicate(),
+      'display' => $field_state['paragraphs'][$delta]['display'],
+      'mode' => 'closed'
+    ];
+
+    static::setWidgetState($parents, $field_name, $form_state, $field_state);
+    $form_state->setRebuild();
+  }
+
   public static function paragraphsItemSubmit(array $form, FormStateInterface $form_state) {
     $button = $form_state->getTriggeringElement();