Join (Add Columns) function duplicates rows

I have 2 tables:

  1. equipments with “username, equipment_name” and total rows of 1300
  2. employees with “username, employee_id”, total rows 100.

When I use the Join (Add Columns) function (Left Join, username = username) to add the employee_id column to the equipments table, the total number of rows suddenly becomes 1400. I can visually confirm duplicates.

This issue seems to be present since 8.3, and persists in the latest version.

I’m running macOS (INTEL), the data was imported from xlxs files.