Fix mapping of enums with duplicate labels

src/EFCore.PG/Infrastructure/Internal/EnumDefinition.cs

@@ -85,10 +85,36 @@ public EnumDefinition(
         ClrType = clrType;
 
         NameTranslator = nameTranslator;
-        Labels = clrType.GetFields(BindingFlags.Static | BindingFlags.Public)
-            .ToDictionary(
-                x => x.GetValue(null)!,
-                x => x.GetCustomAttribute<PgNameAttribute>()?.PgName ?? nameTranslator.TranslateMemberName(x.Name));
+
+        var labels = new Dictionary<object, string>();
+        // Tracks the [PgName] attribute value for each enum value (null if no [PgName] was specified).
+        // Used to detect conflicting [PgName] mappings for different labels that share the same underlying value.
+        var pgNames = new Dictionary<object, string?>();
+        foreach (var field in clrType.GetFields(BindingFlags.Static | BindingFlags.Public))
+        {
+            var value = field.GetValue(null)!;
+            var pgName = field.GetCustomAttribute<PgNameAttribute>()?.PgName;
+
+            if (labels.TryGetValue(value, out _))
+            {
+                var existingPgName = pgNames[value];
+
+                // If either the existing or current field has a [PgName] attribute, they must match.
+                if ((pgName is not null || existingPgName is not null)
+                    && pgName != existingPgName)
+                {
+                    throw new InvalidOperationException(
+                        $"Enum '{clrType.Name}' has multiple members with the same value '{value}' but with different [PgName] mappings ('{existingPgName ?? "(none)"}' and '{pgName ?? "(none)"}'). All members that share the same value must have identical [PgName] mappings.");
+                }
+
+                continue;
+            }
+
+            labels.Add(value, pgName ?? nameTranslator.TranslateMemberName(field.Name));
+            pgNames.Add(value, pgName);
+        }
+
+        Labels = labels;
     }
 
     /// <inheritdoc />

test/EFCore.PG.Tests/Infrastructure/EnumDefinitionTest.cs

@@ -0,0 +1,96 @@
+using Npgsql.EntityFrameworkCore.PostgreSQL.Infrastructure.Internal;
+using Npgsql.NameTranslation;
+
+namespace Npgsql.EntityFrameworkCore.PostgreSQL.Infrastructure;
+
+public class EnumDefinitionTest
+{
+    [ConditionalFact]
+    public void Enum_with_duplicate_values_uses_first_label()
+    {
+        var definition = new EnumDefinition(
+            typeof(EnumWithDuplicateValues),
+            name: null,
+            schema: null,
+            new NpgsqlSnakeCaseNameTranslator());
+
+        Assert.Equal(2, definition.Labels.Count);
+        Assert.Equal("alive", definition.Labels[EnumWithDuplicateValues.Alive]);
+        Assert.Equal("deceased", definition.Labels[EnumWithDuplicateValues.Deceased]);
+    }
+
+    [ConditionalFact]
+    public void Enum_with_duplicate_values_and_same_pg_name_succeeds()
+    {
+        var definition = new EnumDefinition(
+            typeof(EnumWithDuplicateValuesAndSamePgName),
+            name: null,
+            schema: null,
+            new NpgsqlSnakeCaseNameTranslator());
+
+        Assert.Equal(2, definition.Labels.Count);
+        Assert.Equal("custom_alive", definition.Labels[EnumWithDuplicateValuesAndSamePgName.Alive]);
+        Assert.Equal("deceased", definition.Labels[EnumWithDuplicateValuesAndSamePgName.Deceased]);
+    }
+
+    [ConditionalFact]
+    public void Enum_with_duplicate_values_and_different_pg_names_throws()
+    {
+        var exception = Assert.Throws<InvalidOperationException>(
+            () => new EnumDefinition(
+                typeof(EnumWithDuplicateValuesAndDifferentPgNames),
+                name: null,
+                schema: null,
+                new NpgsqlSnakeCaseNameTranslator()));
+
+        Assert.Contains("different", exception.Message, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("[PgName]", exception.Message);
+    }
+
+    [ConditionalFact]
+    public void Enum_with_duplicate_values_where_only_one_has_pg_name_throws()
+    {
+        var exception = Assert.Throws<InvalidOperationException>(
+            () => new EnumDefinition(
+                typeof(EnumWithDuplicateValuesOnePgName),
+                name: null,
+                schema: null,
+                new NpgsqlSnakeCaseNameTranslator()));
+
+        Assert.Contains("different", exception.Message, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("[PgName]", exception.Message);
+    }
+
+    private enum EnumWithDuplicateValues
+    {
+        Alive,
+        Deceased,
+        Comatose = Alive,
+    }
+
+    private enum EnumWithDuplicateValuesAndSamePgName
+    {
+        [PgName("custom_alive")]
+        Alive,
+        Deceased,
+        [PgName("custom_alive")]
+        Comatose = Alive,
+    }
+
+    private enum EnumWithDuplicateValuesAndDifferentPgNames
+    {
+        [PgName("label_a")]
+        Alive,
+        Deceased,
+        [PgName("label_b")]
+        Comatose = Alive,
+    }
+
+    private enum EnumWithDuplicateValuesOnePgName
+    {
+        [PgName("custom_alive")]
+        Alive,
+        Deceased,
+        Comatose = Alive,
+    }
+}