De-duplicate the ordered enabled columns to keep export headers unique

jschick04 · NikTilton · commit d4791c55cfb8 · 2026-06-18T18:25:31.000-05:00
diff --git a/src/EventLogExpert.Runtime/LogTable/LogTableState.cs b/src/EventLogExpert.Runtime/LogTable/LogTableState.cs
@@ -83,8 +83,19 @@ public IReadOnlyList<ColumnName> GetOrderedEnabledColumns(ILogTableColumnDefault
             .ToHashSet();
 
         var order = ColumnOrder.IsEmpty ? columnDefaults.ColumnOrder : ColumnOrder;
-        var ordered = order.Where(enabledColumns.Contains).ToList();
-        var present = ordered.ToHashSet();
+
+        HashSet<ColumnName> present = [];
+        List<ColumnName> ordered = [];
+
+        // De-duplicate while preserving first occurrence: a persisted ColumnOrder may contain duplicates
+        // that would otherwise become duplicate export headers (rejected by TabularExportWriter).
+        foreach (var column in order)
+        {
+            if (enabledColumns.Contains(column) && present.Add(column))
+            {
+                ordered.Add(column);
+            }
+        }
 
         // Append any enabled column missing from the active order (e.g. enabled but absent from a persisted
         // ColumnOrder) so it is never silently dropped from the table or an export.
diff --git a/tests/Unit/EventLogExpert.Runtime.Tests/LogTable/LogTableStateExportTests.cs b/tests/Unit/EventLogExpert.Runtime.Tests/LogTable/LogTableStateExportTests.cs
@@ -34,6 +34,25 @@ public void GetOrderedEnabledColumns_AppendsEnabledColumnsMissingFromOrder()
         Assert.Equal(2, ordered.Count);
     }
 
+    [Fact]
+    public void GetOrderedEnabledColumns_DuplicateOrderAndMissingEnabledColumn_DeduplicatesThenAppends()
+    {
+        var state = new LogTableState
+        {
+            Columns = ImmutableDictionary<ColumnName, bool>.Empty
+                .Add(ColumnName.Source, true)
+                .Add(ColumnName.EventId, true),
+            ColumnOrder = [ColumnName.Source, ColumnName.Source]
+        };
+
+        var ordered = state.GetOrderedEnabledColumns(new ColumnDefaults());
+
+        // Source deduplicated to one entry; EventId (enabled, absent from the order) appended from defaults.
+        Assert.Equal(ColumnName.Source, ordered[0]);
+        Assert.Contains(ColumnName.EventId, ordered);
+        Assert.Equal(2, ordered.Count);
+    }
+
     [Fact]
     public void GetOrderedEnabledColumns_FiltersToEnabled_InColumnOrder()
     {
@@ -51,4 +70,21 @@ public void GetOrderedEnabledColumns_FiltersToEnabled_InColumnOrder()
         ColumnName[] expected = [ColumnName.EventId, ColumnName.Source];
         Assert.Equal(expected, ordered);
     }
+
+    [Fact]
+    public void GetOrderedEnabledColumns_WithDuplicatesInColumnOrder_DeduplicatesPreservingFirstOccurrence()
+    {
+        var state = new LogTableState
+        {
+            Columns = ImmutableDictionary<ColumnName, bool>.Empty
+                .Add(ColumnName.Source, true)
+                .Add(ColumnName.EventId, true),
+            ColumnOrder = [ColumnName.Source, ColumnName.EventId, ColumnName.Source]
+        };
+
+        var ordered = state.GetOrderedEnabledColumns(new ColumnDefaults());
+
+        ColumnName[] expected = [ColumnName.Source, ColumnName.EventId];
+        Assert.Equal(expected, ordered);
+    }
 }
