diff --git a/phpunit/WikisTest.php b/phpunit/WikisTest.php
new file mode 100644
index 0000000..10ae1d6
--- /dev/null
+++ b/phpunit/WikisTest.php
@@ -0,0 +1,41 @@
+<?php
+// use the phpunit framework
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Test Wikis classes
+ */
+final class WikisTest extends TestCase {
+
+	/**
+	 * Test that the wikis can be listed
+	 */
+	public function testListWikisSingleton() {
+
+		$all = [];
+		$wikis = web\MediaWikis::all();
+		foreach( $wikis as $wiki ) {
+
+			$classname = get_class( $wiki );
+
+			// register it once (not called on duplicate)
+			if( !array_key_exists( $classname, $all ) ) {
+				$all[ $classname ] = 0;
+			}
+
+			// increase the counter (called on duplicates)
+			$all[ $classname ]++;
+		}
+
+
+		$n = 0;
+		foreach( $all as $wiki => $count ) {
+			$n += $count;
+		}
+
+		// all the wikis must be presented just once, no more, no less
+		$this->assertEquals( $n, count( $all ) );
+	}
+
+}
+