diff --git a/tests/test_research_backfill_idempotent.py b/tests/test_research_backfill_idempotent.py
new file mode 100644
index 0000000..fac5969
--- /dev/null
+++ b/tests/test_research_backfill_idempotent.py
@@ -0,0 +1,167 @@
+"""Verify research-attribution backfill is replay-safe against real schema.
+
+Three things to prove:
+1. (handle, role, pr_number) with claim_path=NULL deduplicates correctly
+   (idx_ce_unique_pr partial index handles SQLite NULL-not-equal-NULL).
+2. Re-inserting an existing (handle, role, pr_number, NULL) row via INSERT OR IGNORE
+   is a true no-op — does not create a phantom duplicate.
+3. The backfill script's specific operation (DELETE then INSERT for same key)
+   nets zero rows when run twice in sequence.
+"""
+
+import sqlite3
+import sys
+
+# Schema lifted verbatim from lib/db.py:181-209
+SCHEMA = """
+CREATE TABLE contribution_events (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    handle TEXT NOT NULL,
+    kind TEXT NOT NULL DEFAULT 'person',
+    role TEXT NOT NULL,
+    weight REAL NOT NULL,
+    pr_number INTEGER NOT NULL,
+    claim_path TEXT,
+    domain TEXT,
+    channel TEXT,
+    timestamp TEXT NOT NULL DEFAULT (datetime('now'))
+);
+CREATE UNIQUE INDEX idx_ce_unique_claim ON contribution_events(
+    handle, role, pr_number, claim_path
+) WHERE claim_path IS NOT NULL;
+CREATE UNIQUE INDEX idx_ce_unique_pr ON contribution_events(
+    handle, role, pr_number
+) WHERE claim_path IS NULL;
+"""
+
+
+def setup() -> sqlite3.Connection:
+    conn = sqlite3.connect(":memory:")
+    conn.row_factory = sqlite3.Row
+    conn.executescript(SCHEMA)
+    return conn
+
+
+def insert_event(conn, handle, role, pr_number, claim_path=None):
+    cur = conn.execute(
+        """INSERT OR IGNORE INTO contribution_events
+           (handle, kind, role, weight, pr_number, claim_path)
+           VALUES (?, 'agent', ?, 0.30, ?, ?)""",
+        (handle, role, pr_number, claim_path),
+    )
+    return cur.rowcount
+
+
+def count(conn) -> int:
+    return conn.execute("SELECT COUNT(*) FROM contribution_events").fetchone()[0]
+
+
+def test_pr_level_dedup_with_null_claim_path():
+    """Two inserts of same (handle, role, pr_number, NULL) → 1 row."""
+    conn = setup()
+    r1 = insert_event(conn, "rio", "author", 4061)
+    r2 = insert_event(conn, "rio", "author", 4061)
+    n = count(conn)
+    assert r1 == 1, f"first insert should write, got rowcount={r1}"
+    assert r2 == 0, f"second insert should be ignored, got rowcount={r2}"
+    assert n == 1, f"expected 1 row, got {n}"
+    print("PASS: pr-level dedup with NULL claim_path")
+
+
+def test_per_claim_dedup_with_path():
+    """Two inserts of same (handle, role, pr_number, path) → 1 row."""
+    conn = setup()
+    r1 = insert_event(conn, "rio", "author", 4061, claim_path="domains/x.md")
+    r2 = insert_event(conn, "rio", "author", 4061, claim_path="domains/x.md")
+    n = count(conn)
+    assert r1 == 1 and r2 == 0 and n == 1
+    print("PASS: per-claim dedup with claim_path")
+
+
+def test_pr_level_and_per_claim_coexist():
+    """A (handle, role, pr_number, NULL) and (handle, role, pr_number, 'x.md') coexist
+    because the partial indexes target different rows."""
+    conn = setup()
+    r1 = insert_event(conn, "rio", "author", 4061, claim_path=None)
+    r2 = insert_event(conn, "rio", "author", 4061, claim_path="domains/x.md")
+    n = count(conn)
+    assert r1 == 1 and r2 == 1 and n == 2
+    print("PASS: pr-level and per-claim events coexist on same pr_number")
+
+
+def test_backfill_replay_is_noop():
+    """Simulate the exact backfill operation: INSERT correct event, DELETE wrong event.
+    Run twice. Expect identical state — no phantom rows, no double-deletions."""
+    conn = setup()
+
+    # Initial state: m3taversal has the wrong author event for pr=4061
+    insert_event(conn, "m3taversal", "author", 4061)
+    assert count(conn) == 1
+
+    def backfill_pr_4061():
+        # Insert the correct event (rio is the real author)
+        conn.execute(
+            """INSERT OR IGNORE INTO contribution_events
+               (handle, kind, role, weight, pr_number, claim_path)
+               VALUES (?, 'agent', 'author', 0.30, 4061, NULL)""",
+            ("rio (self-directed)",),
+        )
+        # Delete the wrong event
+        conn.execute(
+            """DELETE FROM contribution_events
+               WHERE handle='m3taversal' AND role='author'
+                 AND pr_number=4061 AND claim_path IS NULL""",
+        )
+        conn.commit()
+
+    backfill_pr_4061()
+    state_after_first = sorted(
+        (r["handle"], r["role"], r["pr_number"], r["claim_path"])
+        for r in conn.execute("SELECT * FROM contribution_events")
+    )
+    assert state_after_first == [("rio (self-directed)", "author", 4061, None)], state_after_first
+
+    # Replay
+    backfill_pr_4061()
+    state_after_second = sorted(
+        (r["handle"], r["role"], r["pr_number"], r["claim_path"])
+        for r in conn.execute("SELECT * FROM contribution_events")
+    )
+    assert state_after_first == state_after_second, "replay should be idempotent"
+    assert count(conn) == 1, f"expected 1 row after replay, got {count(conn)}"
+    print("PASS: backfill replay is a true no-op")
+
+
+def test_replay_against_already_backfilled_pr_does_not_double_delete():
+    """If m3taversal event was already deleted, running backfill again must not error
+    or affect anything else."""
+    conn = setup()
+    # Already-correct state: rio has the author event, m3taversal does not
+    insert_event(conn, "rio (self-directed)", "author", 4061)
+    insert_event(conn, "leo", "evaluator", 4061)  # noise — should not be touched
+
+    # Run backfill: tries to INSERT (rio, author, 4061) — already exists, no-op
+    # Tries to DELETE (m3taversal, author, 4061) — already absent, 0 rows affected
+    cur1 = conn.execute(
+        """INSERT OR IGNORE INTO contribution_events
+           (handle, kind, role, weight, pr_number, claim_path)
+           VALUES ('rio (self-directed)', 'agent', 'author', 0.30, 4061, NULL)""",
+    )
+    cur2 = conn.execute(
+        """DELETE FROM contribution_events
+           WHERE handle='m3taversal' AND role='author'
+             AND pr_number=4061 AND claim_path IS NULL""",
+    )
+    assert cur1.rowcount == 0, f"insert should be no-op, got {cur1.rowcount}"
+    assert cur2.rowcount == 0, f"delete should be no-op, got {cur2.rowcount}"
+    assert count(conn) == 2, f"expected 2 rows preserved, got {count(conn)}"
+    print("PASS: replay against already-backfilled state preserves unrelated events")
+
+
+if __name__ == "__main__":
+    test_pr_level_dedup_with_null_claim_path()
+    test_per_claim_dedup_with_path()
+    test_pr_level_and_per_claim_coexist()
+    test_backfill_replay_is_noop()
+    test_replay_against_already_backfilled_pr_does_not_double_delete()
+    print("\nAll 5 tests passed against real schema.")