Nit only add predicate once

```rust
	impl<T> ::parity_scale_codec::MaxEncodedLen for Generic<T>
	where
		T: ::parity_scale_codec::MaxEncodedLen,
```
instead of
```
	impl<T> ::parity_scale_codec::MaxEncodedLen for Generic<T>
	where
		T: ::parity_scale_codec::MaxEncodedLen,
		T: ::parity_scale_codec::MaxEncodedLen,
		T: ::parity_scale_codec::MaxEncodedLen,
```

derive/src/trait_bounds.rs

@@ -161,6 +161,7 @@ pub fn add<N>(
 
 		codec_types
 			.into_iter()
+			.take(1)
 			.for_each(|ty| where_clause.predicates.push(parse_quote!(#ty : #codec_bound)));
 
 		let has_compact_bound: syn::Path = parse_quote!(#crate_path::HasCompact);