Howabout a cooldown from when the same ability could be used on you?

Not as much as the cooldown for a demigod to use his ability again, but just enough to make using 4 sniping Regs a poor strategy.

Gotta wait three seconds before you can snipe a player your teamate just sniped or something like that?  That way its your choice whether or not to use the same demigod as a teamate is using, you just have to live with a slight disadvantage/annoyance, especially if you choose the same build as him.

Axyun, can you give some examples where one and only one DG in a matchup is able to counter some other DG's move?

Well there are some builds that are hard to beat... 5 reguli sniping are very hard to kill... one attacks, all five snipe, you die. You can probably kill two DGs with only one Regulus actually taking the hits, especially if they buy cooldown lowering favor items. But generally, multiple Demigods aren't a problem.

... They're 3 forms of direct damage.  What's to 'counter'?  How are you handling the damage from Fireball differently than the damage from Penitence?

Everyone has access to potions and everyone is able to avoid walking into a gang-beating situation. 

I find that every time my opponents think they are going to be funny and play multiples of the same DG, they get completely rolled. Instead of covering each others' weaknesses, they are all weak to the same thing. Therefore, exploit that weakness and it is easy to defeat a single-DG team.

Assuming equal teamplay skills, a balanced team of different DGs will always defeat one of all the same DG.

What probably happened is that your opponents chose ahead of time to play the same DG, meaning they have teamwork skills. You were playing with random scrubs. They beat you because they were coordinated, but it's likely they could have done the same using mixed DGs as well.

Frankly, if multiple demigods are ganging up on you you're going to have problems.  Doesn't matter whether it's penitence/penitence/penitence or penitence/spit/fireball, you're in trouble.  The only issue I see currently is triple-snipe Regulus team, since they can actually support each other from entirely separate lanes. 

fireballs are close range, so 3 of them can take up to ~3k damage.  also, cooldown is fairly short, allowing for another 3k shot in about 7 secs.  however, only in close range.  if you have an oak, tb and reg, i assume you are going to open with oak goin 1v1, then the fireball, then the oak will chase and use invulnerability to bypass your towers while hitting you with his skills.  then when you think you are safe, you get sniped.  so you are vulnerable the entire time as opposed to being vulnerable close range only with the 3 tb's.

however, to push a lane against 3 tb's, most likely would a team that could get in close, do some interupts and negate their attacks.

or you could go 3 tb vs 3 tb and see who is the best tb evar

I think that the only problem combo is three or more Regulii. Snipe's crazy range allows them to support one another and still be present in multiple flags simultaneously. Snipe can't be interrupted (unless you're very close to the Reg but that's poor performance from the Reg). The only countermeasure is healing or shielding and against a coordinated team you need to get the timing exactly right for it to succeed.

Three Oaks or four TBs or whatevers need to be physically close to one another in order to gank someone and then the situation is no different than any other 3v1 or whatever.

i dont like doubles of any if it is a small match like 3v3 or 4v4 5v5 i can accept 2 dudes having the same but never more