Ensure assets are downloaded just once

Kontent.Statiq.Tests/LinqExtensionsTests.cs

@@ -0,0 +1,29 @@
+using Statiq.Common;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using Xunit;
+
+namespace Kontent.Statiq.Tests
+{
+    public class LinqExtensionsTests
+    {
+        [Fact]
+        public void DistinctRemovesDuplicateEntriesFromASequence()
+        {
+            // Arrange
+            var assets = new List<KontentImageDownload> {
+                new KontentImageDownload("a", new NormalizedPath("/a/")),
+                new KontentImageDownload("a", new NormalizedPath("/a/")),
+                new KontentImageDownload("b", new NormalizedPath("/b/"))
+            };
+
+            // Act
+            var assetUrls = assets.DistinctBy(a => a.LocalPath);
+
+            // Assert
+            Assert.Equal(2, assetUrls.Count());
+            Assert.NotEqual(assetUrls.FirstOrDefault().LocalPath, assetUrls.LastOrDefault().LocalPath);
+        }
+    }
+}

Kontent.Statiq/KontentDownloadImages.cs

@@ -20,7 +20,7 @@ protected override async Task<IEnumerable<IDocument>> ExecuteContextAsync(IExecu
                 .SelectMany(doc => doc.GetKontentImageDownloads())
                 .ToArray();
 
-            var assetUrls = assets.Select(a => a.OriginalUrl).ToArray(); // TODO: distinct by local path
+            var assetUrls = assets.DistinctBy(a => a.LocalPath).Select(a => a.OriginalUrl).ToArray();
 
             WithUris(assetUrls.ToArray());
 

Kontent.Statiq/LinqExtensions.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Collections.Generic;
+
+namespace Kontent.Statiq
+{
+    /// <summary>
+    /// Extension methods enriching the LINQ syntax.
+    /// </summary>
+    public static class LinqExtensions
+    {
+        /// <summary>
+        /// Returns distinct elements from a sequence by using an equality comparer based on specified properties.
+        /// </summary>
+        /// <typeparam name="TSource">The type of the elements of source.</typeparam>
+        /// <typeparam name="TKey">The type of the selector key.</typeparam>
+        /// <param name="source">The sequence to remove duplicate elements from.</param>
+        /// <param name="keySelector">Selector allowing to specify one or more properties to base the filtering on.</param>
+        /// <returns><see cref="IEnumerable{T}"/> that contains distinct elements from the source sequence.</returns>
+        public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
+        {
+            HashSet<TKey> seenKeys = new HashSet<TKey>();
+            foreach (TSource element in source)
+            {
+                if (seenKeys.Add(keySelector(element)))
+                {
+                    yield return element;
+                }
+            }
+        }
+    }
+}