Preserve duplicate column names in query results

Queries returning duplicate column names - e.g. joins between tables
sharing column names - silently lost values because rows were built
with dict(zip(keys, row)), where the last duplicate wins.

Later occurrences are now renamed with a numeric suffix: id, id
becomes id, id_2 - skipping any suffix that would collide with a
real column in the same query.

The new utils.dedupe_keys() helper transforms the key list once per
query, so the per-row dict construction is unchanged and there is no
measurable performance impact.

Applied in Database.query() (including the PRAGMA and RETURNING
paths), Table.rows_where(), Table.search() and the CLI's JSON output.
CSV, TSV and table output keep the original duplicate headers.

Closes #624

tests/test_utils.py

@@ -83,3 +83,20 @@ def test_maximize_csv_field_size_limit():
 )
 def test_flatten(input, expected):
     assert utils.flatten(input) == expected
+
+
+@pytest.mark.parametrize(
+    "input,expected",
+    (
+        ([], []),
+        (["id", "name"], ["id", "name"]),
+        (["id", "id"], ["id", "id_2"]),
+        (["id", "id", "id"], ["id", "id_2", "id_3"]),
+        # A renamed duplicate must not clobber a real column called id_2
+        (["id", "id", "id_2"], ["id", "id_3", "id_2"]),
+        (["id_2", "id", "id"], ["id_2", "id", "id_3"]),
+        (["id", "id", "id_2", "id_2"], ["id", "id_3", "id_2", "id_2_2"]),
+    ),
+)
+def test_dedupe_keys(input, expected):
+    assert utils.dedupe_keys(input) == expected