Remove existing Media Gallery allows you to avoid image duplication issue during the image import, you can use it to add multiple images after product import.

<?php


				$product = Mage::getModel ( 'catalog/product' );
				$product_id = $product->getIdBySku ( $sku);
				$product->load ( $product_id );
				
				/**
				 * BEGIN REMOVE EXISTING MEDIA GALLERY
				 */
				$attributes = $product->getTypeInstance ()->getSetAttributes ();
				if (isset ( $attributes ['media_gallery'] )) {
					$gallery = $attributes ['media_gallery'];
					//Get the images
					$galleryData = $product->getMediaGallery ();
					foreach ( $galleryData ['images'] as $image ) {
						//If image exists
						if ($gallery->getBackend ()->getImage ( $product, $image ['file'] )) {
							$gallery->getBackend ()->removeImage ( $product, $image ['file'] );
						}
					}
					$product->save ();
				}
				/**
				 * END REMOVE EXISTING MEDIA GALLERY
				 */
				try {
					$galleryData = explode ( ';', $value );
					foreach ( $galleryData as $gallery_img ) /**
					 * @param directory where import image resides
					 * @param leave 'null' so that it isn't imported as thumbnail, base, or small
					 * @param false = the image is copied, not moved from the import directory to it's new location
					 * @param false = not excluded from the front end gallery
					 */
					{
						$product->setMediaGallery ( array ('images' => array (), 'values' => array () ) );
						if (file_exists ( Mage::getBaseDir ( 'media' ) . DS . 'import' . $gallery_img )) {
							$product->addImageToMediaGallery ( Mage::getBaseDir ( 'media' ) . DS . 'import' . $gallery_img, array ("thumbnail", "small_image", "image" ), false, false )->save ();
						}
					}
				} catch ( Exception $e ) {
					echo $e->getMessage ();
				}