When there is more than one recommendation call being returned on a specific page, that is when more than one strategy is employed on the same page assumingly on different widgets, it is possible that the second call is not returning products due to the deduplication factor.
This can happen when the two strategies are using the same or similar algorithm.
Here is a case example:
The customer is using both "Similarity" and "Viewed Together" algorithms on the same page.
The first recommendation call is that of "Similarity":
The second strategy is that of "Viewed Together". However, it fell back straight to "Popularity [TOP_N]" and moreover didn't return any products. That is because you can see in the second call, the products are filtered:
The products are filtered because these same products you see in the screenshot above (their groupID) are already returned in the first recommendation.
It is advisable not to employ similar behavioral algorithms on the same page
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