The original version of eZ Reco tracked Node IDs.

In practical use in a content tree where Objects may have multiple Nodes, a fetch for Recommendations may yield the same Object more than once.

This was regarded as a bug by users and reported as such to eZ.

The tracking where then changed to track Object IDs, which is the case for the latest builds for 5.1, 5.0, 4.7 and 4.6.

However, YOOCHOOSE explains that this was in fact intentional; users may use the content differently in different locations in the content tree, therefore the recommendations may be different depending on which page the user is visiting. This also affects how the content Category is tracked, as the URL of the content plays a role in the recommendation algorithm.

Actual result:
Tracking Object ID.

Expected result:
Tracking Node ID.