When planning_horizons = 2030 and grouping_years also contains 2030, p_nom_max for generators is reduced twice by the value of the current installed capacities. This PR ensure to remove duplicated values.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.