[OpenTelemetry] Handle duplicate sampler attributes

src/OpenTelemetry/CHANGELOG.md

@@ -11,6 +11,10 @@ Notes](../../RELEASENOTES.md).
   buffer estimate. Such content is now truncated.
   ([#7543](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7543))
 
+* Fixed activity creation throwing when multiple tracer providers return a
+  sampler attribute with the same key.
+  ([#7558](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7558))
+
 ## 1.17.0
 
 Released 2026-Jul-16

src/OpenTelemetry/Trace/TracerProviderSdk.cs

@@ -492,7 +492,7 @@ private static ActivitySamplingResult ComputeActivitySamplingResult(
             {
                 foreach (var att in attributes)
                 {
-                    options.SamplingTags.Add(att.Key, att.Value);
+                    options.SamplingTags[att.Key] = att.Value;
                 }
             }
         }

test/OpenTelemetry.Tests/Trace/TracerProviderSdkTests.cs

@@ -250,6 +250,40 @@ public void TracerProviderSdkSamplerAttributesAreAppliedToActivity(SamplingDecis
         }
     }
 
+    [Fact]
+    public void TracerProviderSdkDuplicateSamplerAttributesUseLastValue()
+    {
+        const string attributeKey = "duplicate-sampler-attribute";
+        var firstSampler = new TestSampler
+        {
+            SamplingAction = _ => new SamplingResult(
+                SamplingDecision.RecordAndSample,
+                [new(attributeKey, "first")]),
+        };
+        var secondSampler = new TestSampler
+        {
+            SamplingAction = _ => new SamplingResult(
+                SamplingDecision.RecordAndSample,
+                [new(attributeKey, "second")]),
+        };
+
+        var activitySourceName = Utils.GetCurrentMethodName();
+        using var activitySource = new ActivitySource(activitySourceName);
+        using var firstProvider = Sdk.CreateTracerProviderBuilder()
+            .AddSource(activitySourceName)
+            .SetSampler(firstSampler)
+            .Build();
+        using var secondProvider = Sdk.CreateTracerProviderBuilder()
+            .AddSource(activitySourceName)
+            .SetSampler(secondSampler)
+            .Build();
+
+        using var activity = activitySource.StartActivity("root");
+
+        Assert.NotNull(activity);
+        Assert.Equal("second", activity.GetTagItem(attributeKey));
+    }
+
     [Fact]
     public void TracerSdkSetsActivitySamplingResultAsPropagationWhenParentIsRemote()
     {