diff --git a/pkg/smith/lib/builder.dart b/pkg/smith/lib/builder.dart
index 673b0a99965..19b7f2c6b84 100644
--- a/pkg/smith/lib/builder.dart
+++ b/pkg/smith/lib/builder.dart
@@ -99,7 +99,7 @@ class Builder {
   final Architecture arch;
   final Sanitizer sanitizer;
   final Runtime runtime;
-  final List<Configuration> testedConfigurations;
+  final Set<Configuration> testedConfigurations;
 
   Builder(this.name, this.description, this.steps, this.system, this.mode,
       this.arch, this.sanitizer, this.runtime, this.testedConfigurations);
@@ -167,11 +167,11 @@ String _expandVariables(String string, Map<String, String> values) {
   return string;
 }
 
-List<Configuration> _getTestedConfigurations(List<Step> steps) {
+Set<Configuration> _getTestedConfigurations(List<Step> steps) {
   return steps
       .where((step) => step.isTestStep)
       .map((step) => step.testedConfiguration)
-      .toList();
+      .toSet();
 }
 
 T _findIfNotNull<T>(T Function(String) find, String name) {
diff --git a/pkg/smith/lib/test_matrix.dart b/pkg/smith/lib/test_matrix.dart
index 5c8c35d6b0e..3d10b21ff67 100644
--- a/pkg/smith/lib/test_matrix.dart
+++ b/pkg/smith/lib/test_matrix.dart
@@ -56,6 +56,20 @@ class TestMatrix {
     var builders = parseBuilders(builderConfigurations, configurations);
     var branches = <String>[...?json["branches"]];
 
+    // Check that each configuration is tested on at most one builder.
+    var testedOn = <Configuration, Builder>{};
+    for (var builder in builders) {
+      for (var configuration in builder.testedConfigurations) {
+        if (testedOn.containsKey(configuration)) {
+          var other = testedOn[configuration];
+          throw FormatException('Configuration "${configuration.name}" is '
+              'tested on both "${builder.name}" and "${other.name}"');
+        } else {
+          testedOn[configuration] = builder;
+        }
+      }
+    }
+
     return TestMatrix._(configurations, builders, branches);
   }
 
diff --git a/pkg/smith/test/test_matrix_test.dart b/pkg/smith/test/test_matrix_test.dart
index 0884e67d43a..9787567f562 100644
--- a/pkg/smith/test/test_matrix_test.dart
+++ b/pkg/smith/test/test_matrix_test.dart
@@ -120,6 +120,35 @@ void main() {
         ]
       });
     });
+
+    test("a configuration is tested on more than one builder", () {
+      expectJsonError(
+          'Configuration "fasta-linux" is tested on both '
+          '"test-fasta-2" and "test-fasta-1"',
+          {
+            "configurations": {"fasta-linux": {}},
+            "builder_configurations": [
+              {
+                "builders": ["test-fasta-1"],
+                "steps": [
+                  {
+                    "name": "fasta1",
+                    "arguments": [r"-nfasta-linux"],
+                  },
+                ],
+              },
+              {
+                "builders": ["test-fasta-2"],
+                "steps": [
+                  {
+                    "name": "fasta2",
+                    "arguments": [r"-nfasta-linux"],
+                  },
+                ],
+              },
+            ]
+          });
+    });
   });
 
   test("a list of branches is parsed", () {