Option to overwrite rather than merge HashMaps

I have multiple config sources. For most data types, when a field is available from multiple sources, there's an order of precedence for which one to use. However, one of the fields in my settings struct is a HashMap. I have found that when this field is available from multiple config settings, the HashMaps are merged together, where keys from both sources are present in the deserialized output. I would prefer that one whole HashMap overrides another, to bring the behavior in line with that of the primitive types. Or, at least have an option I can call on the ConfigBuilder to ask for this behavior.

Start by locating ConfigBuilder and tracing how HashMaps from multiple config sources are combined, then inspect the surrounding precedence behavior. Define completion as an available option that makes one whole HashMap override another rather than merging keys, with the resulting deserialized settings matching the selected precedence.