This is probably controversial, but let me explain. Dual fruits are not always overpowered. Naturally, they shall be monitored for things like godmodding and the like. There are a few rules with using two fruits.
1. Dual Fruits CAN be used in conjunction, if it works.
2. Dual fruit users cannot use Haoshoku haki, or Rokushiki. The weakness to water and seastone is doubled as well.
3. To obtain dual fruits, one MUST find one of them ICly, regardless of whether you started with one or not.
4. All Dual fruit users will be monitored heavily for any thing considered to be godmod.
5. Dual fruit users cannot have their fruits be conflicting. Ie, goro goro and gomu gomu could not be in the same body.
For all fruit;
1. Fruits cannot be overpowered in nature, obviously. Remember, if you arent sure, just ask.
2. Fruits can be gained IC by either someone with a fruit dying nearby and their fruit reincarnating itself into another fruit or by finding a fruit.
3. Seastone and water will half all stats, and nullify active DF powers.
4. Fruits, when someone with one is killed, will spawn onto the nearest normal fruit, changing it into a devil fruit.