Merge group members when same group is defined across multiple files

[melipass]

If a group group-name is defined in two different files, the group members won't be combined into a single group definition. This will throw an error:

duplicate name "group:group-name" in object (lines X:Y and Z:Y)

I believe it is expected that a combiner does the following if a group name is encountered multiple times:

1. Collect all members from each occurrence
2. Merge them into a single group definition (deduplicating members, if needed)
3. Then, include the group in the final combined output

Groups with the same name defined across multiple files should have their members merged into a single group definition in the combined output. Merging groups would enable much more flexible organizational structures in child files.

Example

companies/company-1/groups.hujson:

{
    "groups": {
        "group:group-name": [
            "member-1@company.com",
            "member-2@company.com",
        ]
    }
}

companies/company-2/groups.hujson:

{
    "groups": {
        "group:group-name": [
            "member-3@company.com",
            "member-4@company.com",
        ]
    }
}

Expected combined output:

{
    "groups": {
        // from `companies/company-1/groups.hujson`
        // and  `companies/company-2/groups.hujson`
        "group:group-name": [
            "member-1@company.com",
            "member-2@company.com",
            "member-3@company.com",
            "member-4@company.com",
        ]
    }
}

Current output:

{
    "groups": {
        // from `companies/company-1/groups.hujson`
        "group:group-name": ["member-1@company.com", "member-2@company.com"],
        // from `companies/company-2/groups.hujson`
        "group:group-name": ["member-3@company.com", "member-4@company.com"],
    }
}