diff --git a/src/Weekdays.php b/src/Weekdays.php
index 87232d3..3282327 100644
--- a/src/Weekdays.php
+++ b/src/Weekdays.php
@@ -13,7 +13,13 @@ public function __construct(
         /** @var array<int, Weekday> $weekdays */
         public array $weekdays,
     ) {
-        // TODO: Validate for unique
+        $enumValues = [];
+        foreach ($this->weekdays as $weekday) {
+            $enumValues[] = $weekday->value;
+        }
+        if (count($enumValues) !== count(array_unique($enumValues))) {
+            throw new \InvalidArgumentException('Weekdays must be unique.');
+        }
     }
 
     // -- Array normalizable
diff --git a/tests/Weekdays/ConstructionTest.php b/tests/Weekdays/ConstructionTest.php
new file mode 100644
index 0000000..2a7625f
--- /dev/null
+++ b/tests/Weekdays/ConstructionTest.php
@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DigitalCraftsman\DateTimePrecision\Weekdays;
+
+use DigitalCraftsman\DateTimePrecision\Weekday;
+use DigitalCraftsman\DateTimePrecision\Weekdays;
+use PHPUnit\Framework\TestCase;
+
+/** @coversDefaultClass \DigitalCraftsman\DateTimePrecision\Weekdays */
+final class ConstructionTest extends TestCase
+{
+    /**
+     * @test
+     *
+     * @covers ::__construct
+     *
+     * @doesNotPerformAssertions
+     */
+    public function construction_works(): void
+    {
+        // -- Act & Assert
+        new Weekdays([
+            Weekday::MONDAY,
+            Weekday::TUESDAY,
+        ]);
+    }
+
+    /**
+     * @test
+     *
+     * @covers ::__construct
+     */
+    public function construction_fails(): void
+    {
+        // -- Assert
+        $this->expectException(\InvalidArgumentException::class);
+
+        // -- Act
+        new Weekdays([
+            Weekday::MONDAY,
+            Weekday::MONDAY,
+            Weekday::TUESDAY,
+        ]);
+    }
+}