Multi-select fields don't add items, they overwrite

Multi-select fields don't add items, they overwrite

Hello,

For Action Leads/insertRecords?

When I choose duplicateCheck =2, it finds the same email, and updates the record. Fine. But don't you think that all Multi-select fields should have Items added to the list, rather then replacing all the options already selected, with the ONE new one? That should be the default. 

How do I fix this issue? 

Thanks