There isn't any. There are some consoles that you can only gain the effect of one, like the one that increases your shield capacity, or the Assimilated console, but for the most part, adding another console will give you the same benefit as earlier ones.
The Diminishing returns happens due to the basic math that is used for calculating the final numbers. For example, armor/hull resistance works like this: net damage = base damage / ( 1 + (<Armor/resistance total> / 100 ) ). To make it simple. To reduced damage by 1/2, you need 100 pts of armor/resistance (i.e. X / ( 1 + (100 / 100) )). To reduced a damage by another 1/2 again, you need +200 pts of armor/resistance.
Cryptic will sometimes add more factors to make the diminishing return curve a bit steeper like with the Accuracy / Defense curve. But it only happens at the extreme ends of the curves. Normally you won't see any diminishing returns at all.
And another thing to remember, it's the total that counts towards diminishing returns. It doesn't matter how you get there (i.e. numbers of consoles, numbers of stacking buffs, etc)