diff --git a/wres-config/src/wres/config/yaml/DeclarationFactory.java b/wres-config/src/wres/config/yaml/DeclarationFactory.java
index 785835495..83e9a6759 100644
--- a/wres-config/src/wres/config/yaml/DeclarationFactory.java
+++ b/wres-config/src/wres/config/yaml/DeclarationFactory.java
@@ -47,6 +47,7 @@
 import org.yaml.snakeyaml.LoaderOptions;
 import org.yaml.snakeyaml.Yaml;
 import org.yaml.snakeyaml.constructor.Constructor;
+import org.yaml.snakeyaml.constructor.DuplicateKeyException;
 import org.yaml.snakeyaml.nodes.Tag;
 import org.yaml.snakeyaml.parser.ParserException;
 import org.yaml.snakeyaml.representer.Representer;
@@ -418,7 +419,7 @@ static JsonNode deserialize( String yaml ) throws IOException
             // Deserialize with Jackson now that any anchors/references are resolved
             return DESERIALIZER.readTree( resolvedYamlString );
         }
-        catch ( ScannerException | ParserException | JsonProcessingException e )
+        catch ( ScannerException | ParserException | JsonProcessingException | DuplicateKeyException e )
         {
             throw new IOException( "Failed to deserialize a YAML string.", e );
         }
diff --git a/wres-config/src/wres/config/yaml/components/Features.java b/wres-config/src/wres/config/yaml/components/Features.java
index aab319103..170f7fbd9 100644
--- a/wres-config/src/wres/config/yaml/components/Features.java
+++ b/wres-config/src/wres/config/yaml/components/Features.java
@@ -31,6 +31,7 @@ public record Features( Set<GeometryTuple> geometries, Map<GeometryTuple,Offset>
     /**
      * Sets the default values.
      * @param geometries the geometries
+     * @param offsets the offset values, such as a datum offset, if any
      */
     public Features
     {
diff --git a/wres-config/src/wres/config/yaml/components/Threshold.java b/wres-config/src/wres/config/yaml/components/Threshold.java
index 8788fb9ad..14182057d 100644
--- a/wres-config/src/wres/config/yaml/components/Threshold.java
+++ b/wres-config/src/wres/config/yaml/components/Threshold.java
@@ -38,6 +38,7 @@ public record Threshold( wres.statistics.generated.Threshold threshold,
      * @param type the threshold type to help identify the declaration context
      * @param feature a feature
      * @param featureNameFrom the orientation of the data from which the named feature is taken
+     * @param generated true if this threshold was generated by the software, false if declared or read from a source
      */
     public Threshold
     {
@@ -65,6 +66,7 @@ public String toString()
                 .append( "thresholdType", this.type() )
                 .append( "featureName", featureString )
                 .append( "featureNameFrom", this.featureNameFrom() )
+                .append( "generated", this.generated() )
                 .build();
     }
 }
diff --git a/wres-config/test/wres/config/yaml/DeclarationFactoryTest.java b/wres-config/test/wres/config/yaml/DeclarationFactoryTest.java
index cdc2b69fd..760996ef9 100644
--- a/wres-config/test/wres/config/yaml/DeclarationFactoryTest.java
+++ b/wres-config/test/wres/config/yaml/DeclarationFactoryTest.java
@@ -27,6 +27,7 @@
 
 import static org.junit.jupiter.api.Assertions.assertAll;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import wres.config.MetricConstants;
 import wres.config.yaml.components.AnalysisTimes;
@@ -2274,6 +2275,36 @@ void testDeserializeWithCovariates() throws IOException
         assertEquals( expected, actual );
     }
 
+    @Test
+    void testDeserializeThrowsExpectedExceptionWhenDuplicateKeysEncountered()
+    {
+        // GitHub issue #336
+        String yaml = """
+                observed:
+                  - some_file.csv
+                predicted:
+                  sources: another_file.csv
+                probability_thresholds: [0.01,0.1,0.5,0.9,0.95,0.99,0.995,0.999]
+                probability_thresholds: [0.01,0.1,0.5,0.9,0.95,0.99,0.995,0.999]
+                """;
+
+        assertThrows( IOException.class, () -> DeclarationFactory.from( yaml ) );
+    }
+
+    @Test
+    void testDeserializeThrowsExpectedExceptionWhenSpacingIncorrect()
+    {
+        String yaml = """
+                observed:
+                  - some_file.csv
+                predicted:
+                  sources: another_file.csv
+                          probability_thresholds: [0.01,0.1,0.5,0.9,0.95,0.99,0.995,0.999]
+                """;
+
+        assertThrows( IOException.class, () -> DeclarationFactory.from( yaml ) );
+    }
+
     @Test
     void testSerializeWithShortSources() throws IOException
     {