src/types/class-papi-attachment-type.php
public function singleton() {
		$key = sprintf( 'entry_type_id.post_type.%s', $this->get_post_type() );

		if ( papi()->exists( $key ) ) {
			return true;
		}

		papi()->singleton( $key, $this->get_id() );

		return false;
	}
public function singleton() {
		$key = sprintf( 'entry_type_id.post_type.%s', $this->get_post_type() );

		if ( papi()->exists( $key ) ) {
			return true;
		}

		papi()->singleton( $key, $this->get_id() );

		return false;
	}
Check if the entry type is a singleton.

@return bool