Lets get a couple things straight before we move on with this discussion:
Let's define "counter"
As Soffacet said before, several pages ago, each demigod has multiple builds. So with demigod X, you have Xa, Xb, and Xc.
(For example, Spit Beast, Ooze Beast, and Hybrid Beast. Which one is better overall is moot to this discussion.)
Xa, Xb, and Xc all do well in different situations, i.e. Xa may be good against demigod Ya, and Xb may be good against demigod Yb. This holds true for a good percentage of situations in demigod. But there is a problem, demigod is not as simple as 1 v 1.
The only serious games to consider are games 2 v 2 and above, and below 5 v 5. Doubles at 5 v 5 is far too frequent, as there is only 8 demigods, plus the existence of 3 other demigods on the team offsets the "doubling" i think.
Xa may beat Ya flat out, but there are an astronomical number of other factors that go into each engagement between demigods. There are teammates, items, experience levels, citadel upgrades, creep presence, etc. So even though Xa may beat Ya on the open field with no flags, items, creeps, etc, no single engagement will ever be like that.
There are so many ways to fluctuate the "balance" of the engagement that one can say that there is no solid counter to any single demigod out there (again, barring a pure 1 v 1 nothing else engagement). So essentially, even if you have Xa, Ya shouldn't be ready to hit the concede button the first time he fights.
Ok, so hopefully i made sense in showing that no single build can guaruntee a defeat of another build 100% of the time. I do grant that builds do have strengths over others, but high level players can work around these.
The other thing is that doubling up Xa, so essentialy Xa + Xa, does not net you some "extra" demigod power. Adding together two Demigod Defining Abilities (DGDA's reffered to earlier), does not give you some magical multiplier in their strength. Also, adding together two strengths leaves the other team open to 2 times their weaknesses. For example, if you went up against a double spit beast, they would have twice the harrasment power! But, they would also lack the close in melee damage that ooze offers, and they would be far squishier. Versus twin ooze beast, which would leave them very very weak to range harrasment.
Xa + Xa /= nXa where n > 2
Xa + Xa = 2Xa always.
Twin sniping regulus, for example, grants them insane sniping damage! but they BOTH lack what the other builds give them.
Essentially you become twice as powerful against what you already were good against as that 1 reg, but you become twice as weak against what you were already weak against.
Also, people, you cannot say that "OMG. TWO LEVEL 15 SNIPES DOES INSANE DAMGE!!! ITS SO OVERPOWERED!!!" Well shit. Its two level 15 snipes? wtf do you expect? Hopefully you have some level 15 skills of your own that do good?
So, double demigods? Should they be banned permanantly? No, of course not.
Double demigods, can you ban them from your custom games? Sure, go ahead, its your game.
Double demigods, are they overpowered? Sure, if you pick the things they are good against, and fail to realize it and adjust your strategy.
Please, do not flame here. We are trying to have a serious discussion. lets keep it friendly 