--- /dev/null
+<?php
+
+namespace Drupal\aggregator\Plugin\Validation\Constraint;
+
+use Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint;
+
+/**
+ * Supports validating feed URLs.
+ *
+ * @Constraint(
+ * id = "FeedUrl",
+ * label = @Translation("Feed URL", context = "Validation")
+ * )
+ */
+class FeedUrlConstraint extends UniqueFieldConstraint {
+
+ public $message = 'A feed with this URL %value already exists. Enter a unique URL.';
+
+}