So why do you assume that multi-owner ship plays no a role ? I have atleast anecdotal evidence and I said I could be wrong. But you just think it doesnt play a role. Your point is as strong or weak as mine. 

Most people I know who have the systems have had multiple DSes (Lite/i upgrade!), multiple PSPs (2000/3000 upgrade!), multiple 360s (RROD!) and multiple PS3s (YLOD! slim upgrade!).  Strangely though, they've almost all only had one Wii.

I didn't assume anything, I said "I doubt". It could be a bigger factor but I doubt it.

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Yes you made a good point. The Wii is the perfect example the Wii didnt change much except recently the black one it had no hardware failures and no newly designed versions. But all othe gaming platforms have them.

Nintendo WANTS to sell new Nintendo DS to existing owners thats why they make those improvments the market are also existing owners. I mean why should someone buy a DSi if he wasnt interested in the DS ? And there is a special group which  buy three or more especially in japan were people love new hardware revisions and collect them.

Lets speculate alittle:

5% of the Userbase bought atleast 3 DS over the years that means that 5% people accounts for 15% of all DS That means that 1.5 Million User bought 4.5 Million DS so we have just 25.5 Million left (its all just speculation)
Ok lets say that 10 Million DS user have one DS in Japan 1.5 Million have 3 so we have 14.5 Million the rest has 2.

50,5% of the people (10 Million people) have ONE DS

7.5% (1.5 Million) have three DS

42% (8.25 Million) would have two DS

So we have for example 19.75 Million Owners thats almost 1/5 of the population which makes sense to me.

And Nintendos strategy would have worked. Its just an example but we know that Nintendo tries to sell their DS through improvments to existing owners thats why I doubt that for the Wii multi-owner ship plays yet a role because its reliable and the it has never seen an improvment.

Xbox360 has RRODS 

PS3 YLOD and Slim 

PSP has 2000/3000

Homeconsoles are probably to expensive yet so multiowner ship will play a bigger role  later I think. Especially Handhelds are seen by the public and they are also a Lifestyle item and people pay a certain attention to what model it is.

I just want to show you that it could be also the other way around we dont know nothing yet but I remeber reading an article about multiple DS selling to the same person and they stated how clever Nintendo is and sells even more consoles to a saturated market.

Sure, but you're sort of skirting over my previous point... when DS owners "upgrade" they tend to recirculate their old system, and not just stuff it in a drawer or throw it in the garbage.  They trade it in or (more often according to Iwata) hand it down to a younger sibling, parent, spouse, friend, etc.  Iwata even talked about this at length, and said they view traditional gamers as a trojan horse help to introduce their products to a more expanded audience (this happens on Wii too btw, though not in the same exact way).  "Multiownership" probably can't account for figures even as high as you're suggesting... even though a good amount of people may have purchased multiple DSes, those DSes likely still aren't in the ownership of just that person and as a result the active userbase is still notably higher.

The reason Nintendo comes up with DS -> DS Lite -> DSi -> DSi XL is because they're following the Blue Ocean Strategy. If you read it, it discusses the plan for who you target in order. First former customers, ie. lapsed gamers. The DS was first released with a demo of a first person shooter, something that couldn't properly be done on handhelds before. The plan is to reignite some excitement in them with something new. Then you target people who are on the verge, people who are open to your product but aren't quite interested enough. The DS Lite was designed with the look and feel of an ipod with this purpose in mind, and software like Brain Training was released to make the console more relevant to their lives. Third you go after the hard to reach customers, people who don't yet know or care about your product. The DSi and DSi XL are made with this intention in mind. It's designed as a 'tool', which is exactly what BOS says to do, but one that puts those tools into the normal context of your product. So rather than just having a music player, a camera etc. they are all designed to be about 'play'. In addition to that they both have bigger screens than the DS Lite, and a different kind of pen, because the market that is the toughest to reach is older people.

I don't need a second Wee.

Hmm this cant be decided if we dont have numbers. I still think that there are maybe 20 Million different people in Japan which have a Nintendo DS people collect it. People break them accidentialy people have one in the car and one at home and so on. Selling maybe is an option to some but I see it like cellphones people have multiple cellphones also some might have multiple DS. And in Japan families are actually shrinking. Not everyone has people to give the old stuff further.

So I say userbase is 20 Million and not 30 because of Multiple owner and probably broke consoles, you are disagreeing the points are good but aslong as we dont have a good survey about it we will never know the truth.

Attributing 10m out of 30m to broken or collected figures is still way too extreme imo.  I'm sorry but if a third of Japanese DS units are out of circulation/use, then it has nearly the best tie ratio in history... considering it's a handheld (which have traditionally LOW tie ratios, and HIGH piracy) I'd say this is pretty much impossible.

I'd venture ~5m is probably the max figure, and honestly, even that's pushing it.

Nice.