Handle duplicate static web asset identities gracefully

src/StaticWebAssetsSdk/Tasks/Compression/DiscoverPrecompressedAssets.cs

@@ -32,7 +32,36 @@ 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.TryGetValue(asset.Identity, out var existing))
+            {
+                if (existing.SourceId != asset.SourceId ||
+                    existing.RelativePath != asset.RelativePath)
+                {
+                    Log.LogWarning(
+                        "Duplicate candidate asset '{0}' with differing metadata (SourceId='{1}' vs '{2}', RelativePath='{3}' vs '{4}'). Keeping first occurrence.",
+                        asset.Identity,
+                        existing.SourceId,
+                        asset.SourceId,
+                        existing.RelativePath,
+                        asset.RelativePath);
+                }
+                else
+                {
+                    Log.LogMessage(
+                        MessageImportance.Low,
+                        "Skipping duplicate candidate asset '{0}' (SourceId='{1}'). Assets are identical.",
+                        asset.Identity,
+                        asset.SourceId);
+                }
+            }
+            else
+            {
+                candidatesByIdentity[asset.Identity] = asset;
+            }
+        }
 
         foreach (var candidate in candidates)
         {

src/StaticWebAssetsSdk/Tasks/Data/StaticWebAsset.cs

@@ -9,6 +9,7 @@
 using System.Security.Cryptography;
 using Microsoft.AspNetCore.StaticWebAssets.Tasks.Utils;
 using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
 
 namespace Microsoft.AspNetCore.StaticWebAssets.Tasks;
 
@@ -1228,13 +1229,41 @@ internal static FileInfo ResolveFile(string identity, string originalItemSpec)
         throw new InvalidOperationException($"No file exists for the asset at either location '{identity}' or '{originalItemSpec}'.");
     }
 
-    internal static Dictionary<string, StaticWebAsset> ToAssetDictionary(ITaskItem[] candidateAssets, bool validate = false)
+    internal static Dictionary<string, StaticWebAsset> ToAssetDictionary(
+        ITaskItem[] candidateAssets,
+        bool validate = false,
+        TaskLoggingHelper log = null)
     {
         var dictionary = new Dictionary<string, StaticWebAsset>(candidateAssets.Length);
         for (var i = 0; i < candidateAssets.Length; i++)
         {
             var candidateAsset = FromTaskItem(candidateAssets[i], validate);
-            dictionary.Add(candidateAsset.Identity, candidateAsset);
+            if (dictionary.TryGetValue(candidateAsset.Identity, out var existing))
+            {
+                if (existing.SourceId != candidateAsset.SourceId ||
+                    existing.RelativePath != candidateAsset.RelativePath)
+                {
+                    log?.LogWarning(
+                        "Duplicate static web asset '{0}' with differing metadata (SourceId='{1}' vs '{2}', RelativePath='{3}' vs '{4}'). Keeping first occurrence.",
+                        candidateAsset.Identity,
+                        existing.SourceId,
+                        candidateAsset.SourceId,
+                        existing.RelativePath,
+                        candidateAsset.RelativePath);
+                }
+                else
+                {
+                    log?.LogMessage(
+                        MessageImportance.Low,
+                        "Skipping duplicate static web asset '{0}' (SourceId='{1}'). Assets are identical.",
+                        candidateAsset.Identity,
+                        candidateAsset.SourceId);
+                }
+            }
+            else
+            {
+                dictionary.Add(candidateAsset.Identity, candidateAsset);
+            }
         }
 
         return dictionary;

src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetEndpointsManifest.cs

@@ -64,8 +64,37 @@ 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.TryGetValue(a.ResolvedAsset.Identity, out var existing))
+                {
+                    if (existing.TargetPath != a.TargetPath ||
+                        existing.ResolvedAsset.SourceId != a.ResolvedAsset.SourceId)
+                    {
+                        Log.LogWarning(
+                            "Duplicate static web asset '{0}' with differing metadata (TargetPath='{1}' vs '{2}', SourceId='{3}' vs '{4}'). Keeping first occurrence.",
+                            a.ResolvedAsset.Identity,
+                            existing.TargetPath,
+                            a.TargetPath,
+                            existing.ResolvedAsset.SourceId,
+                            a.ResolvedAsset.SourceId);
+                    }
+                    else
+                    {
+                        Log.LogMessage(
+                            MessageImportance.Low,
+                            "Skipping duplicate static web asset '{0}' (SourceId='{1}'). Assets are identical.",
+                            a.ResolvedAsset.Identity,
+                            a.ResolvedAsset.SourceId);
+                    }
+                }
+                else
+                {
+                    manifestAssets[a.ResolvedAsset.Identity] = a;
+                }
+            }
 
             // Build exclusion matcher if patterns are provided
             StaticWebAssetGlobMatcher exclusionMatcher = null;

src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetsManifest.cs

@@ -99,7 +99,36 @@ 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.TryGetValue(a.Identity, out var existing))
+                {
+                    if (existing.SourceId != a.SourceId ||
+                        existing.RelativePath != a.RelativePath)
+                    {
+                        Log.LogWarning(
+                            "Duplicate static web asset '{0}' with differing metadata (SourceId='{1}' vs '{2}', RelativePath='{3}' vs '{4}'). Keeping first occurrence.",
+                            a.Identity,
+                            existing.SourceId,
+                            a.SourceId,
+                            existing.RelativePath,
+                            a.RelativePath);
+                    }
+                    else
+                    {
+                        Log.LogMessage(
+                            MessageImportance.Low,
+                            "Skipping duplicate static web asset '{0}' (SourceId='{1}'). Assets are identical.",
+                            a.Identity,
+                            a.SourceId);
+                    }
+                }
+                else
+                {
+                    assetsByIdentity[a.Identity] = a;
+                }
+            }
             var filteredEndpoints = new List<StaticWebAssetEndpoint>();
 
             foreach (var endpoint in Endpoints.Select(StaticWebAssetEndpoint.FromTaskItem))