Without rehashing things said in other threads, I believe the number of duals allowed on any one account needs to be limited by Game rule to 2 duals inaddition to the account holder. Let me first say that for a few years I have played dual accounts (one dual) exclusively myself, and I would not advocate prohibiting duals altogether for more than one reason. For one thing, players such as myself who depend on a dual to help run an account with help from sitters would not play at all without a dual. The player base is already so diminished that further loss of players would be disastrous.
I have no suggestion as to how such a rule could be enforced, but am confident there are measures in place or available to be put in place which could detect when more than three players have logged into the account with the account password, or that it could be coded in to give each account up to three sets of unique login credentials (email and password). Then sharing passwords could be prohibited, and you would have no more than three players, total, on any one account. ( I am too old and too dadburned grumpy to play solo anymore, so please don't go there).
I am also a gold user, and my dual also contributes gold to our accounts. However, when an account has upwards of six players on it, all contributing gold, then gold use can indeed get out of hand and give a group of sub-par players an extreme advantage over other players on a server - especially when that server has the "incorporate troops" feature like the SE server, UK20. Troops, both offense and defense, can be trained in several villages, sent as reinforcement to one village and for gold (or resources) then incorporated into that village. The incorporated troops then take on the upgrade level of the village they were incorporated into and can be used as if trained in that village. Six duals and an account holder can easily afford a huge pile of gold when all seven contribute. This can be a big problem, and should not be possible under game rules as it puts all other players who do not take the same advantage of what I see as a loophole at a gigantic disadvantage in effect breaking the server.