SerializerBuilder / BuiltList - generated code contains duplicated builder factories

Hi

If I have multiple models with a list of types or nullable types, it creates things like this:
```dart

// **************************************************************************
// BuiltValueGenerator
// **************************************************************************

Serializers _$serializers = ...
..addBuilderFactory(
const FullType(BuiltList, const [const FullType(int)]),
() => new ListBuilder())
..addBuilderFactory(
const FullType(BuiltList, const [const FullType(int)]),
() => new ListBuilder())
..addBuilderFactory(
const FullType(BuiltList, const [const FullType(int)]),
..addBuilderFactory(
const FullType(BuiltList, const [const FullType(String)]),
() => new ListBuilder())
..addBuilderFactory(
const FullType(BuiltList, const [const FullType.nullable(int)]),
() => new ListBuilder())
..addBuilderFactory(
const FullType(BuiltList, const [const FullType.nullable(int)]),
() => new ListBuilder())
..addBuilderFactory(
const FullType(BuiltList, const [const FullType(String)]),
() => new ListBuilder())
```

If you check the `BuiltList`, `BuiltList` and `BuiltList` serializers are duplicated:

# Environment:
- `built_value_generator: 8.3.2`
- `build_runner: 2.1.5`
- `built_value: 8.3.2`
- `built_collection: 5.1.1`