Yes, there is a stacking penalty. The description doesn't mention it, but it exists. Do keep in mind though that the stacking penalty only applies to the modules. So don't apply any penalty to bonus from the scout suit, the bonus from the actual dampening skill, or the first module. Penalties start with the second modules.

Something that is pretty nice it. Profile damps lvl 5 = standard active scanner can not find your logi or assault suit, no modules needed.

oh hi your the noob I've seen around the forum a lot latly please stop all the hissy fits it's getting annoying.

me I'm nobody just a lurker who is tired of seeing you complain and try and wreak the game have a nice day :D thanks for insulting my corp I didn't know them back in mag but I'll be sure to tell them that they suck now and that they were never any good thank you very much for pointing it out to me.

of course I can do something I can ask you to stop. that doesn't mean you will stop but I've still done something about it.
by the way the whole bolding of sentences is getting old too.

orders who is giving orders? I'm asking not telling telling would be foolish as there would be no way for me to enforce it. asking on the other hand is compleatly different and entirely feasible that someone would listen to a request from a stranger over the internet. please get your words right.

No they don't

trolling you oh you think your important enough for me to troll, cute, I ask only for you to stop trying to speak your idiotic ideals as there are some easily influenced people out there who might actually listen to you. have a good night.

Wow, you guys just don't stop with proliferating wrong information.

How many of you sent a TICKET to CCP and asked if stacking penalties applied to profile dampeners? I did, and they said they DID apply.

How many of you have profile dampening to Level 5 and have discovered that you CAN'T avoid 28 dB scans by using one Complex Dampener and one Enhanced Dampener on a medium suit? I have. I MUST use 2 complex to avoid proto scanners.

By your logic:

50dB * .90 (skill at level 5) = 45 dB
45 dB * .75 (complex) = 33.75 dB
33.75 dB * .80 (enhanced) = 27 dB (should avoid proto scans, but it doesn't)

This is what really happens:

50 dB * .90 (skill at level 5) = 45 dB
45 dB * .6262 (complex and enhanced using stacking penalty calculator) = 28.18 dB (doesn't avoid proto scans)

With two complex dampeners:
50dB * .90 (skill at level 5) = 45 dB
45 dB * .587 (two complex using stacking penalty calculator) = 26.42 dB (avoids proto scans)

And oh, KING CHECKMATE, FYR you only need two militia or basic profile dampeners to avoid advanced scans if you have level 3 in the skill (your profile is 34.74 dB). There is no added benefit in using higher than basic/militia in two slots at your skill level, since you can't go under proto scans unless you use three slots for three enhanced or get your dampening to proto.