The notifier role was first added to this play in in December 2014 [1].
This PR was primarily notifier-related.
See: [3]

This role was again added, this time without a configured interval, in
February of 2015 [2]. This role was added alongside several others.
Perhaps the duplication was overlooked at the time?
See: [4]

[1] https://github.com/edx/configuration/commit/83053b52863e230d38ee0c2c2b2a71601725fe90
[2] https://github.com/edx/configuration/commit/cef9193ddf7b5c450be44d55e4ffd620a9e73392
[3] https://github.com/edx/configuration/blob/141ef9d2fd3c4788b04309e55c15f9de03adb030/playbooks/edx_sandbox.yml#L59
[4] https://github.com/edx/configuration/blob/141ef9d2fd3c4788b04309e55c15f9de03adb030/playbooks/edx_sandbox.yml#L49