I am using the new (release group) series variables and am writing the name _releasegroup_series and the number attribute _releasegroup_seriesnumber to tags.

Each time when I refresh the release, these variables (and hence their corresponding tags) contain (another) duplicate of the same data, e.g. three times the same value(s) after two refreshes.

As this is happening neither with the new _lyricistsort variable nor in my previous plugin implementation of series tags, I've checked the implementation for differences:

The series implementation is using metadata.add while the sort name implementation (and many other tags) are using metadata.add_unique.

Since it could theoretically happen that a release/release group/recording is part of multiple series with the same name or number attribute, using metadata.add_unique might not be a good solution in this case. It would probably make sense to use an approach as in my plugin where I overwrite the whole tags with all their new values at once.

Whatever the decision is, it would also make sense to revisit other tags and variables which could legally contain duplicate values and double-check them.