Handle duplicate static web asset identities gracefully

src/StaticWebAssetsSdk/Tasks/Compression/DiscoverPrecompressedAssets.cs

@@ -32,7 +32,21 @@ public override bool Execute()
         var candidates = StaticWebAsset.FromTaskItemGroup(CandidateAssets);
         var assetsToUpdate = new List<ITaskItem>();
 
-        var candidatesByIdentity = candidates.ToDictionary(asset => asset.Identity, OSPath.PathComparer);
+        var candidatesByIdentity = new Dictionary<string, StaticWebAsset>(OSPath.PathComparer);
+        foreach (var asset in candidates)
+        {
+            if (!candidatesByIdentity.ContainsKey(asset.Identity))
+            {
+                candidatesByIdentity[asset.Identity] = asset;
+            }
+            else
+            {
+                Log.LogMessage(
+                    MessageImportance.Low,
+                    "Skipping duplicate candidate asset '{0}'. The asset was already added from a different source.",
+                    asset.Identity);
+            }
+        }
 
         foreach (var candidate in candidates)
         {

src/StaticWebAssetsSdk/Tasks/Data/StaticWebAsset.cs

@@ -1230,11 +1230,14 @@
 
     internal static Dictionary<string, StaticWebAsset> ToAssetDictionary(ITaskItem[] candidateAssets, bool validate = false)
     {
-        var dictionary = new Dictionary<string, StaticWebAsset>(candidateAssets.Length);
+        var dictionary = new Dictionary<string, StaticWebAsset>(candidateAssets.Length, OSPath.PathComparer);
         for (var i = 0; i < candidateAssets.Length; i++)
         {
             var candidateAsset = FromTaskItem(candidateAssets[i], validate);
-            dictionary.Add(candidateAsset.Identity, candidateAsset);
+            if (!dictionary.ContainsKey(candidateAsset.Identity))
+            {
+                dictionary.Add(candidateAsset.Identity, candidateAsset);
+            }
         }
 
         return dictionary;

src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetEndpointsManifest.cs

@@ -64,8 +64,15 @@ public override bool Execute()
 
             // Get the list of the asset that need to be part of the manifest (this is similar to GenerateStaticWebAssetsDevelopmentManifest)
             var assets = StaticWebAsset.FromTaskItemGroup(Assets);
-            var manifestAssets = ComputeManifestAssets(assets, ManifestType)
-                .ToDictionary(a => a.ResolvedAsset.Identity, a => a, OSPath.PathComparer);
+            var manifestAssetsList = ComputeManifestAssets(assets, ManifestType);
+            var manifestAssets = new Dictionary<string, TargetPathAssetPair>(OSPath.PathComparer);
+            foreach (var a in manifestAssetsList)
+            {
+                if (!manifestAssets.ContainsKey(a.ResolvedAsset.Identity))
+                {
+                    manifestAssets[a.ResolvedAsset.Identity] = a;
+                }
+            }
 
             // Build exclusion matcher if patterns are provided
             StaticWebAssetGlobMatcher exclusionMatcher = null;

src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetsManifest.cs

@@ -99,7 +99,14 @@ private StaticWebAssetEndpoint[] FilterPublishEndpointsIfNeeded(StaticWebAsset[]
         // inside the manifest because its cumbersome to do it in MSBuild directly.
         if (StaticWebAssetsManifest.ManifestTypes.IsPublish(ManifestType))
         {
-            var assetsByIdentity = assets.ToDictionary(a => a.Identity, a => a, OSPath.PathComparer);
+            var assetsByIdentity = new Dictionary<string, StaticWebAsset>(OSPath.PathComparer);
+            foreach (var a in assets)
+            {
+                if (!assetsByIdentity.ContainsKey(a.Identity))
+                {
+                    assetsByIdentity[a.Identity] = a;
+                }
+            }
             var filteredEndpoints = new List<StaticWebAssetEndpoint>();
 
             foreach (var endpoint in Endpoints.Select(StaticWebAssetEndpoint.FromTaskItem))