In Runn, you can only add one assignment per day per resource per role on the same project.
If you are on the Premium or Enterprise plan, you can use Workstreams to add assignments to the same person under different phases.
Another way to get around this is to add them twice to the same project- but under different roles.
Create a new role in Manage > Roles with the correct rates.
Add the resource twice - once under each role.
Create assignments for the resource under both roles and add details or assign to separate phases.
Here's a short video of how: