fix(import): collapse duplicate fee-type warnings into a bounded list

test/mv/membership/import/import_runner_test.exs

@@ -21,6 +21,65 @@ defmodule Mv.Membership.Import.ImportRunnerTest do
     end
   end
 
+  describe "merge_progress/4 warning accumulation" do
+    test "deduplicates identical warnings across chunks instead of growing unbounded" do
+      progress = %{
+        inserted: 0,
+        failed: 0,
+        errors: [],
+        warnings: ["Fee type 'Ghost' not found; using the default fee type."],
+        status: :running,
+        current_chunk: 0,
+        total_chunks: 3
+      }
+
+      chunk_result = %{
+        inserted: 2,
+        failed: 0,
+        errors: [],
+        errors_truncated?: false,
+        warnings: [
+          "Fee type 'Ghost' not found; using the default fee type.",
+          "Fee type 'Ghost' not found; using the default fee type."
+        ]
+      }
+
+      result = ImportRunner.merge_progress(progress, chunk_result, 0)
+
+      assert result.warnings == ["Fee type 'Ghost' not found; using the default fee type."]
+    end
+
+    test "preserves distinct warnings while collapsing duplicates" do
+      progress = %{
+        inserted: 0,
+        failed: 0,
+        errors: [],
+        warnings: ["Fee type 'A' not found; using the default fee type."],
+        status: :running,
+        current_chunk: 0,
+        total_chunks: 2
+      }
+
+      chunk_result = %{
+        inserted: 1,
+        failed: 0,
+        errors: [],
+        errors_truncated?: false,
+        warnings: [
+          "Fee type 'A' not found; using the default fee type.",
+          "Fee type 'B' not found; using the default fee type."
+        ]
+      }
+
+      result = ImportRunner.merge_progress(progress, chunk_result, 0)
+
+      assert result.warnings == [
+               "Fee type 'A' not found; using the default fee type.",
+               "Fee type 'B' not found; using the default fee type."
+             ]
+    end
+  end
+
   describe "read_file_entry/2" do
     test "returns {:ok, content} for a readable file" do
       path =