Task cohort_role_sync attempts to sync users and fails when two cohorts have a common user.

Steps to replicate

1. Create a Cohort 1
2. Create a Cohort 2
3. Add a user X to the Cohort 1
4. Add the same user X to the Cohort 2
5. Create a Role which can be assigned at the user level
6. Go to Site administration / ► Users / ► Permissions / ► Assign user roles to cohort
7. Choose a user Y from the 'Select users to assign role' dropdown
8. Select the Cohort 1 and Cohort 2 from the 'Select cohorts' dropdown
9. Click Assign
10. Run the scheduled task (php admin/tool/task/cli/schedule_task.php --execute=tool_cohortroles\\task
    cohort_role_sync --showdebugging)

You will get the next message:

++ Did you remember to make the first column something unique in your call to get_records? Duplicate value '8' found in column 'userid'. ++