Check for duplicate keys in input data

Raise an error if two rate items have the same name. E.g, this would
trigger an error:

```
- name: Test Rate
  history:
    - values: "1""
      from: 2023-06
      until: 2023-11
- name: Test Rate
  history:
    - values: "2""
      from: 2023-12
```

src/nerc_rates/models.py

@@ -54,9 +54,18 @@ def validate_no_overlap(cls, data: Self):
         return data
 
 
+def check_for_duplicates(items):
+    data = {}
+    for item in items:
+        if item["name"] in data:
+            raise ValueError(f"found duplicate name \"{item['name']}\" in list")
+        data[item["name"]] = item
+    return data
+
+
 RateItemDict = Annotated[
     dict[str, RateItem],
-    pydantic.BeforeValidator(lambda items: {x["name"]: x for x in items}),
+    pydantic.BeforeValidator(check_for_duplicates),
 ]
 
 

src/nerc_rates/tests/test_rates.py

@@ -82,3 +82,25 @@ def test_rates_get_value_at():
     assert r.get_value_at("Test Rate", "2021-01") == "2"
     with pytest.raises(ValueError):
         assert r.get_value_at("Test Rate", "2019-01")
+
+
+def test_fail_with_duplicate_names():
+    with pytest.raises(ValueError):
+        rates.Rates(
+            [
+                {
+                    "name": "Test Rate",
+                    "history": [
+                        {"value": "1", "from": "2020-01", "until": "2020-12"},
+                        {"value": "2", "from": "2021-01"},
+                    ],
+                },
+                {
+                    "name": "Test Rate",
+                    "history": [
+                        {"value": "1", "from": "2020-01", "until": "2020-12"},
+                        {"value": "2", "from": "2021-01"},
+                    ],
+                },
+            ]
+        )