What limitations for Weapon Attachments exist, exactly? I know you can only take one scope, but could you (Technically) buy multiples of the same attachment? I know it would be nearly irrelevant for most upgrades, (Since nearly all of them just apply some kind of passive buff (For example, it doesn't matter how many extra grips you have, you'll still only be using one,) but I mainly ask because of Exterminators.

Since I'm not reading any technical limitations, is there anything stopping someone from buying like ten Exterminators and building them into a gun? Besides the cost (Just buy a flamer!) and Weight (JUST BUY A FLAMER!)

Pretty sure it's just the one instance of a particular attachment/upgrade/modification.

Or, if that isn't explicitly spelled out, it should be.

If it is, technically, permissible (or, at least, not prohibited) by RAW, I would houserule that you cannot have duplicate attachments/modifications/upgrades on the same weapon (unless they are operating on component parts of a combi-weapon, or similar). IE, you can normallyl only have one fire selector or on a gun, but if the weapon is something like the Spectre or Puritan-14 from Inquisitor's handbook, the primary and secondary weapon components can each be upgraded with a fire selector.

Definitly something to discuss with the GM. Most of the same time I would say no, but the PC might have a convincing reason, or has even produced some cool artwork or a model which demonstrates the idea. This is WH40K so things can be ramped up somewhat. But as I say, most of the time, I think one attachment is enough.

The only limitation that I know of existing is the aforementioned "Only one Scope"-rule. There is nothing by RAW, in any of the Core Rulebooks at least, prohibiting multiples of the same upgrade, as far as I am aware.

That being said, you really need to take it up with the GM. While it's never explicitly stated that you are limited to one Upgrade of the same kind, this is akin to making a peasant railgun work. Interpreting RAW for the sake of RAW is funny, but it never actually works in play, simply because "Hah, no.".

About the "Only 1 Scope"-rule, by the way, I always found it terrible. Most often, there is nothing preventing a weapon from having multiple scopes. The rule really should be that you can only use 1 Scope at a time, but your weapon can have as many as is reasonable (for example, why would you not be able to have a Telescopic Sight and a Laser Sight at the same time? Stacking their use wouldn't make sense, but switching to the Telescopic Sight when Aiming makes complete sense).