It is something done in the data manager. It creates unique field names by adding the table name as a 'prefix' to the field name. However, the field name 'Stream' is already reserved in your case. You can do one of three things:
1. Use the data load editor and just load the tables normally - they will concatenate themselves on the common field name(s).
2. Rename the Association name at the bottom of that screen in your picture to Stream1 (for example).
3. Change the Stream column names in your original data sets to Stream1 and Stream2 (for example) and rename the association to Stream at the bottom of that screen in your picture.