Implemented an overlap detection module in the postprocessing chain

internal/config/config.go

@@ -137,6 +137,7 @@ func normalizeInputFiles(paths []string) ([]string, error) {
 	}
 
 	normalized := make([]string, 0, len(paths))
+	seen := make(map[string]struct{}, len(paths))
 	for _, path := range paths {
 		path = strings.TrimSpace(path)
 		if path == "" {
@@ -147,6 +148,10 @@ func normalizeInputFiles(paths []string) ([]string, error) {
 		if err := requireFile(clean, "--input-file"); err != nil {
 			return nil, err
 		}
+		if _, exists := seen[clean]; exists {
+			return nil, fmt.Errorf("duplicate --input-file %q", clean)
+		}
+		seen[clean] = struct{}{}
 		normalized = append(normalized, clean)
 	}
 	sort.Strings(normalized)

internal/config/config_test.go

@@ -3,6 +3,7 @@ package config
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -24,6 +25,27 @@ func TestOmittingNormalizeSpeakersDoesNotRequireSpeakers(t *testing.T) {
 	}
 }
 
+func TestDuplicateInputFilesFailValidation(t *testing.T) {
+	dir := t.TempDir()
+	input := writeTempFile(t, dir, "input.json")
+	output := filepath.Join(dir, "merged.json")
+
+	_, err := NewMergeConfig(MergeOptions{
+		InputFiles:            []string{input, input},
+		OutputFile:            output,
+		InputReader:           DefaultInputReader,
+		OutputModules:         DefaultOutputModules,
+		PreprocessingModules:  DefaultPreprocessingModules,
+		PostprocessingModules: DefaultPostprocessingModules,
+	})
+	if err == nil {
+		t.Fatal("expected duplicate input error")
+	}
+	if !strings.Contains(err.Error(), "duplicate --input-file") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 func writeTempFile(t *testing.T, dir string, name string) string {
 	t.Helper()