common: group mounts by uuid and scrub only the first

sort + groupby fanciness by @klausenbusk.

roles/common/tasks/main.yml

@@ -37,8 +37,10 @@
 
 - name: start and enable btrfs scrub timer
   service: name=btrfs-scrub@{{ '-' if (item.mount | length == 1) else (item.mount.split("/", 1)[1] | replace("/", "-")) }}.timer enabled=yes state=started
-  loop: "{{ ansible_mounts }}"
-  when: "item.fstype == 'btrfs' and item.mount != '/var/lib/docker/btrfs' and 'backup' not in item.mount"
+  loop: "{{ ansible_mounts | sort(attribute='mount') | groupby('uuid') | map(attribute=1) | map('first') }}"
+  when:
+    - item.fstype == 'btrfs'
+    - not 'backup' in item.mount
 
 - name: install mlocate
   pacman: name=mlocate state=present