The way stacking penalties work in EVE ( and Dust, you can check the stacking penalties against Cardiac and Kinetics and they will follow this pattern. ) is like this:

For positive modifiers:
Base * Skill Modifier * Skill Modifier * (1+Module_Modifier) * (1+Module_Modifier*Stacking Penalty) * (1+Module_Modifier*Stacking Penalty)GǪ

For negative ( reductions ) modifiers:
Base * Skill Modifier * Skill Modifier * (1-Module_Modifier) * (1-Module_Modifier*Stacking Penalty) * (1-Module_Modifier*Stacking Penalty)GǪ

The stacking penalty increasing for each additional module.

The stacking penalty can be approximated by = 0.5^(((n-1)/2.22292081)^2) where n is the number of the module. This makes the modules work with the following efficacy.
1st module = 100%
2nd Gëê 86.91%
3rd Gëê 57.06%
4th Gëê 28.30%
5th Gëê 10.60%

If we apply this to a medium suit, which has a base scan profile of 50 dB, we get the following:

Base = 50 dB
Skill Modifier = 2% reduction in scan profile per level of 'Profile Dampening' (10% reduction at level 5)
Complex Profile Dampener = 25% reduction in scan profile
Enhanced Profile Dampener = 20% reduction in scan profile
Standard Profile Dampener = 15% reduction in scan profile

So a medium suit with level 5 in profile dampening and 2 complex profile dampeners will have:
50 * 0.9 * (1-0.25) * (1-0.25*0.8691) Gëê 26.42 dB scan profile

This is clearly lower than the 28 dB scan precision of a prototype active scanner and the suit will not be detected.

However if you have a medium suit with level 5 in profile dampening and 1 complex profile dampener and 1 enhanced profile dampener you will get the following:
50 * 0.9 * (1-0.25) * (1-0.20*0.8691) Gëê 27.88 dB scan profile

This is also clearly lower than the 28 dB scan precision of a prototype active scanner and the suit should not have been detected, how ever it is detected. ( we tried a few different prototype scanners and all of them detect this suit. )

Now we assume the reason for it being detected is because of the "armour bug".

In game:
Complex armour plate has a movement penalty of 5%.
Complex ferroscale plate has NO movement penalty how ever we assume it actually has one that is set to 0%. ( The calculation will show why we think this. )
Medium suit: 5.00 m/s
1st module, complex armour plate: 4.75 m/s
2nd module, complex armour plate: 4.54 m/s

Now the weird bit, we put on a ferroscale plate.

3rd module, complex ferroscale plate: 4.64 m/s

A few quick calculations to show stacking penalties:
5.00 * (1-0.05) = 4.75
5.00 * (1-0.05) * (1-0.05*0.8691) Gëê 4.54

Now for the weird bit:
5.00 * (1-0) * (1-0.05*0.8691) * (1-0.05*0.5706) Gëê 4.64

As you can see here both the assumption that the ferroscale actually has a movement penalty of 0% and that it is calculated 'first in line' in the stack is confirmed.

Now supposedly EVE ( and Dust ) are to apply the largest modifiers first and then the smaller ones. This leads us back to the example of a medium suit with 1 complex- & 1 enhanced profile dampener being detected by a prototype active scanner. You see if you apply the stacking penalty in the "wrong" order you will get:
50 * 0.9 * (1-0.20) * (1-0.25*0.8691) Gëê 28.18 dB scan profile

Which would explain why the suit is, currently, being detected by a prototype active scanner.

Me and a corpmate did a bunch of testing with active and passive scanning and yes 'Precision Enhancers', 'Profile Dampeners' and 'Range Amplifiers' all suffer from stacking penalties.

Edit 1: I do not know if anyone got a confirmation that this is a bug from a dev, to be fair I do not remember anyone saying that they heard back from any dev regarding this. I am certain, how ever, that it was reported back when ferroscales were introduced.

Edit 2: It was confirmed as a defect by 'CCP Cmdr Wang' back in july. Link to weekly bug reports: https://forums.dust514.com/default.aspx?g=posts&m=1057188#post1057188 - '[BUG] Ferroscale speed bonus?'

'xxwhitedevilxx M' reported the bug on the forums in the begining of july: https://forums.dust514.com/default.aspx?g=posts&m=1009079#post1009079