db.clients.aggregate([
  { $group: {
      _id: { companyId: "$companyId", email: { $toLower: "$email" } },
      count: { $sum: 1 },
      emails: { $addToSet: "$email" }
  } },
  { $match: { count: { $gt: 1 }, $expr: { $gt: [{ $size: "$emails" }, 1] } } }
])