Duplicate Facet Summary Items

I can confirm this issue, and it still applies for 2.x and 3.x.

This happens when using hierarchical facets and having a taxonomy term active which has multiple parents.
In this case DefaultFacetsSummaryManager::build will go through the results tree in buildResultTree and add any active result. Since it uses array_merge and there is no duplicates detection (e.g: that could be determined by the raw_value) we get multiple facets summary items.

h4. Suggestion:
We could simply use the raw_value as key and use the array union (+) operator instead in buildResultTree.
Then in build we can leave array_merge (since 1 facet's raw_value could be the same as another facet's raw_value) but this can be changed to use the spread operator instead for performance.

The last submitted patch, 3: facet_summary_hierarchy_duplicates-3161773-3-TEST-ONLY-FAIL.patch, failed testing. View results →

I found an issue with patch #3: if we have the same raw value between facets then we get only 1 facet summary item.
This new patch uses "facet_ID:raw_value" as array keys to prevent duplicates and uses the + operator everywhere, which is more efficient.