diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index f8094d6b3d..1b78760e97 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -4,6 +4,7 @@ Yii Framework 2 Change Log
 2.0.14 under development
 ------------------------
 
+- Enh #15047: `yii\db\Query::select()` and `yii\db\Query::addSelect()` now check for duplicate column names (wapmorgan)
 - Enh #14643: Added `yii\web\ErrorAction::$layout` property to conveniently set layout from error action config (swods, cebe, samdark)
 - Enh #13465: Added `yii\helpers\FileHelper::findDirectory()` method (ArsSirek, developeruz)
 - Enh #8527: Added `yii\i18n\Locale` component having `getCurrencySymbol()` method (amarox, samdark)
diff --git a/framework/db/Query.php b/framework/db/Query.php
index cec4d36497..896709e465 100644
--- a/framework/db/Query.php
+++ b/framework/db/Query.php
@@ -594,7 +594,7 @@ PATTERN;
         } elseif (!is_array($columns)) {
             $columns = preg_split('/\s*,\s*/', trim($columns), -1, PREG_SPLIT_NO_EMPTY);
         }
-        $this->select = $columns;
+        $this->select = $this->getUniqueColumns($columns);
         $this->selectOption = $option;
         return $this;
     }
@@ -621,6 +621,7 @@ PATTERN;
         } elseif (!is_array($columns)) {
             $columns = preg_split('/\s*,\s*/', trim($columns), -1, PREG_SPLIT_NO_EMPTY);
         }
+        $columns = $this->getUniqueColumns($columns);
         if ($this->select === null) {
             $this->select = $columns;
         } else {
@@ -630,6 +631,51 @@ PATTERN;
         return $this;
     }
 
+    /**
+     * Returns unique column names excluding duplicates.
+     * Columns to be removed:
+     * - if column definition already present in SELECT part with same alias
+     * - if column definition without alias already present in SELECT part without alias too
+     * @param array $columns the columns to be merged to the select.
+     * @since 2.0.14
+     */
+    protected function getUniqueColumns($columns)
+    {
+        $columns = array_unique($columns);
+        $unaliasedColumns = $this->getUnaliasedColumnsFromSelect();
+
+        foreach ($columns as $columnAlias => $columnDefinition) {
+            if ($columnDefinition instanceof Query) {
+                continue;
+            }
+
+            if (
+                (is_string($columnAlias) && isset($this->select[$columnAlias]) && $this->select[$columnAlias] === $columnDefinition)
+                || (is_integer($columnAlias) && in_array($columnDefinition, $unaliasedColumns))
+            ) {
+                unset($columns[$columnAlias]);
+            }
+        }
+        return $columns;
+    }
+
+    /**
+     * @return array List of columns without aliases from SELECT statement.
+     * @since 2.0.14
+     */
+    protected function getUnaliasedColumnsFromSelect()
+    {
+        $result = [];
+        if (is_array($this->select)) {
+            foreach ($this->select as $name => $value) {
+                if (is_integer($name)) {
+                    $result[] = $value;
+                }
+            }
+        }
+        return array_unique($result);
+    }
+
     /**
      * Sets the value indicating whether to SELECT DISTINCT or not.
      * @param bool $value whether to SELECT DISTINCT or not.
@@ -1180,4 +1226,13 @@ PATTERN;
             'params' => $from->params,
         ]);
     }
+
+    /**
+     * Returns the SQL representation of Query
+     * @return string
+     */
+    public function __toString()
+    {
+        return serialize($this);
+    }
 }
diff --git a/tests/framework/db/QueryTest.php b/tests/framework/db/QueryTest.php
index 6448a35942..5ddda5d5b3 100644
--- a/tests/framework/db/QueryTest.php
+++ b/tests/framework/db/QueryTest.php
@@ -37,6 +37,30 @@ abstract class QueryTest extends DatabaseTestCase
         $query->select('id, name');
         $query->addSelect('email');
         $this->assertEquals(['id', 'name', 'email'], $query->select);
+
+        $query = new Query();
+        $query->select('name, lastname');
+        $query->addSelect('name');
+        $this->assertEquals(['name', 'lastname'], $query->select);
+
+        $query = new Query();
+        $query->addSelect(['*', 'abc']);
+        $query->addSelect(['*', 'bca']);
+        $this->assertEquals(['*', 'abc', 'bca'], $query->select);
+
+        $query = new Query();
+        $query->addSelect(['field1 as a', 'field 1 as b']);
+        $this->assertEquals(['field1 as a', 'field 1 as b'], $query->select);
+
+        $query = new Query();
+        $query->select(['name' => 'firstname', 'lastname']);
+        $query->addSelect(['firstname', 'surname' => 'lastname']);
+        $query->addSelect(['firstname', 'lastname']);
+        $this->assertEquals(['name' => 'firstname', 'lastname', 'firstname', 'surname' => 'lastname'], $query->select);
+
+        $query = new Query();
+        $query->select('name, name, name as X, name as X');
+        $this->assertEquals(['name', 'name as X'], array_values($query->select));
     }
 
     public function testFrom()