Guard against duplicate xunit attachment names

src/Verify.XunitV3.Tests/AttachmentTests.cs

@@ -42,5 +42,27 @@ void Delete()
         Assert.EndsWith(".received.txt", key);
         Assert.DoesNotContain(':', key);
     }
+
+    [Fact]
+    public async Task DuplicateAttachmentName()
+    {
+        DiffEngine.BuildServerDetector.Detected = true;
+        var verified = Path.GetFullPath(CurrentFile.Relative("AttachmentTests.DuplicateAttachmentName.verified.txt"));
+        File.Delete(verified);
+        await File.WriteAllTextAsync(verified, "expected");
+        var settings = new VerifySettings();
+        settings.DisableRequireUniquePrefix();
+
+        // Both failing verifies share one received file name, so it is added as an attachment
+        // twice. Without the dedup guard in AddFile the second add throws ArgumentException
+        // instead of the expected VerifyException.
+        await Assert.ThrowsAsync<VerifyException>(() => Verify("one", settings));
+        await Assert.ThrowsAsync<VerifyException>(() => Verify("two", settings));
+
+        File.Delete(verified);
+
+        var key = Assert.Single(TestContext.Current.Attachments!).Key;
+        Assert.EndsWith(".received.txt", key);
+    }
 #endif
 }

src/Verify.XunitV3/Verifier.cs

@@ -3,10 +3,20 @@ namespace VerifyXunit;
 
 public static partial class Verifier
 {
-    static async Task AddFile(string path) =>
-        TestContext.Current.AddAttachment(
-            GetAttachmentName(path),
-            await File.ReadAllBytesAsync(path));
+    static async Task AddFile(string path)
+    {
+        var name = GetAttachmentName(path);
+        var context = TestContext.Current;
+        // A single test can produce the same received file name from multiple failing
+        // Verify calls (e.g. when DisableRequireUniquePrefix is used). xunit throws on a
+        // duplicate attachment name, so skip if it has already been added.
+        if (context.Attachments?.ContainsKey(name) == true)
+        {
+            return;
+        }
+
+        context.AddAttachment(name, await File.ReadAllBytesAsync(path));
+    }
 
     internal static string GetAttachmentName(string path)
     {