[{field:[["Bob","Mary"],["Bob","Mark"],["Mary","Bob"],["Mary","Mark"],["Mark","Bob"],["Mark","Mary"],["Andrew","Mary"],["Andrew","Mark"],["Mary","Andrew"],["Mary","Mark"],["Mark","Andrew"],["Mark","Mary"],["Bob","Mark"],["Mark","Bob"]]}]

db.collection.aggregate([{$project:{field:{$map:{input:{$setUnion:"$field"},in:["$$this",{$size:{$filter:{input:"$field",as:"a",cond:{$eq:["$$this","$$a"]}}}}]}}}}])