duplicate records with the same email across lead partitions can be affected by uploaded custom object activity
System not separating lead partitions for custom object imports
Uploading custom object activity affects duplicate records across partitions
Custom Object activity import writing to the most recently updated record regardless of partition
Use the following workaround:
Run an update on the records that should be affected by the custom object activities import. This will make sure they are the most recently updated records among the duplicates
Normally when you do a list import, it would import into the one partition, whatever partition the list is located in and only impact the record in that one instance. But when you import a Custom Object, it's able the see ALL the records across ALL the partitions. If there are duplicate records, it just looks to see which one was updated most recently and grabs that one for the list import.
So if you have lead record in A with Add to Custom Object already completed and you re-import again but the lead record in B has been updated more recently and has NOT had the custom object added to it, you get the duplicate "Add to Custom Object" activity.
Multiple lead partitions with duplicate records across them