Initial addition results with duplicates:
sd 0 (size 4), Content light level metadata
sd 1 (size 4), H.26[45] User Data Unregistered SEI message: 1
sd 2 (size 4), H.26[45] User Data Unregistered SEI message: 2
sd 3 (size 4), H.26[45] User Data Unregistered SEI message: 3
sd 4 (size 4), Spherical Mapping
sd 5 (size 4), H.26[45] User Data Unregistered SEI message: 4
sd 6 (size 4), H.26[45] User Data Unregistered SEI message: 5
sd 7 (size 4), H.26[45] User Data Unregistered SEI message: 6

Final state after a single 'no-duplicates' addition:
sd 0 (size 4), Content light level metadata
sd 1 (size 4), Spherical Mapping
sd 2 (size 4), H.26[45] User Data Unregistered SEI message: 1337