Why are rows duplicating in PHP array with array merge function? -

i trying create add cart functionality. using php array_merge function merge values cart. rows duplicating in array. need solve issue.

if(isset($_session["cart_item"])) {     if(is_array($_session["cart_item"])){         if(array_key_exists($productbycode[0]["code"],$_session["cart_item"])) {             foreach($_session["cart_item"] $k => $v) {                 if($productbycode[0]["code"] == $_session["cart_item"][$k]['code']){                     $_session["cart_item"][$k]["quantity"] = $_session["cart_item"][$k]["quantity"] + $_post["quantity"];                     break;                   }             }         } elseif(!array_key_exists($productbycode[0]["code"],$_session["cart_item"])  && !in_array($productbycode[0]["name"], $_session["cart_item"])) {             $_session["cart_item"] = array_merge($_session["cart_item"],$itemarray);         }      }    }elseif(!isset($_session["cart_item"])){     $_session["cart_item"] = $itemarray; }