I'm confused over how to get Necrosha in collections. There seems to be a HC that covers all from New Mutants, X-Force and Legacy, but those series also seem to have Necrosha tie in collections, which are the same issues. E.g. X-Force v4: Necrosha. I like the idea of the oversized complete HC, as it would sit nice next to the Messiah Complex, Endangered Species, etc... ones on my shelf, but don't want to shoot myself in the foot (i.e. end up having the same thing twice) over future collections of New Mutants, X-Force and Legacy. 
 
TIA

The hardback collects  X-Necrosha (one-shot), X-Force 21-25, New Mutants 6-8 & X-Men: Legacy 231-234.  That's every issue involved in the cross-over bar Necrosha: The Gathering. 
 
They wont reprint those issues in other trades.  Messiah War was a cross over between Cable & X-Force.  But it was reprinted as a separate trade, not as an X-Force specific story.  Therefore, you can buy X-Force hardcovers up until Suicide Leper, and the next volume jumps straight into Not Forgotten.... missing Messiah War.

You could have provided that info to begin with. 
 
So if you know what's in all the volumes, then what's the problem?  You don't want to buy it all over again...?  Then don't buy it.  

thats simple. sell your single issues either to a store or online and buy the HC.

problem being the HC seems to be the only place the legacy issues are collected and if I buy it I'm duping stuff. 
seems a really confusing way to collect things imo

Amazon sometimes get the issue numbers wrong.  So, dont worry. If you're planing to just the x-force one, it will have x-nercosha one shot and #21 - 25. 

Im not talking about monthly single issues. I mean TPB.

I was just wondering about the necrosha storyline and the collection myself. X-Force has a trade that collects the issues that were involve in the story. So i'm guessing its better to get it in the X-Necrosha trade. I recently read Second Coming (AWESOME STORY) and X-Force 26-28 were in it and continued in other titles. Like in X-Force 27, the team goes off to take out Master Mold and then in the following issue, it was Uncanny X-Men 525 where they started to kick everyone's butt. X-Force 27 was Chapter 9 but X-Force 28 was Chapter 13. So if someone read 27 then 28, I think they be lost.

Necrosha seems to alternate between X-Force, then X-men Legacy, and maybe New Mutants then X-Force again.