Within the article group, one or more features contain identical feature indexes.
Explanation:
Using the reference position (HeaderIndex1..X) you can specify the order of the features within one article group.
Within an article group must apply:
all HeaderIndexes are empty or do not exist.
or
there are one or more filled HeaderIndex columns present.
In the latter case, a HeaderIndex must be present and filled in for each feature of an article group. The completed indexes must form an ascending sequence of numbers. Double and empty indexes must not be present, otherwise the order cannot be determined.
Solution:
Empty all feature positions (HeaderIndex1..X), or make sure the feature positions all have a value and do not have double values.
