[mappings] minor fix

Author: capcom6

internal/commands/messages/batch/mappings/mappings.go

@@ -44,6 +44,9 @@ func ParseColumnMapping(raw string) (map[string]string, error) {
 		if _, ok := allowed[key]; !ok {
 			return nil, fmt.Errorf("%w: unsupported mapping field %q", ErrMappingParseFailed, key)
 		}
+		if _, exists := result[key]; exists {
+			return nil, fmt.Errorf("%w: duplicate mapping field %q", ErrMappingParseFailed, key)
+		}
 
 		result[key] = column
 	}