How do Lead Assignment rules effect DupeBlocker Warning Tasks, evaluation of DupeBlocker Filters and owners of the records involved in the event of an Auto-Merge/Convert?
Lead Assignment rules execute AFTER the DupeBlocker scenario is evaluated. If "Match on Insert" is set to "Auto-Merge/Auto-Convert", the assignment rule is evaluated BEFORE the "Auto-Merge/Convert" happens (as this triggers an asynchronous apex call to perform the merge/convert as called for in the scenario). In other words, the lead assignment rule will be invoked AFTER the Lead is inserted but BEFORE it is auto-merged/converted.
DupeBlocker scenarios with "Report Duplicate" which create warning tasks for current user, "Yes - Current User", will assign the task to the existing owner of the Lead (not the new owner based on the assignment rule).
Any DupeBlocker Filters based on the Lead Owner field will use the existing owner of the Lead (not the new owner based on the assignment rule).
When "Auto-Merge/Convert" is selected, the Lead Owner that is retained on the merged record will depend on the "Auto-Merge/Convert Settings". The default setting for the Owner field is "update if blank", in which case the existing Lead Owner for the Master Record will be retained. If this is changed to "Overwrite", then the existing owner will be overwritten with the new owner based in the Lead Assignment rule.