I have multiple of some kits, simply for doing variants... Like the Monogram B-29, you can't have just one, you need three at a minimum.. One from the 509th, a regular B-29A, and then a Korean War B-29.. Four or five would be better though, to add the YB-29 and the KB-29 or SB-29 varnants.
Need at least three Monogram B-17Gs too... One 8th AF, one US Navy PB-1, and an RAF Fortress.
Three Libs, for the B-24D, B-24J, and a PB4Y-1..
8th, 9th, 12th, and 15th AF P-51Ds.. Plus variants of the P-51B, A-36, etc..
You get the idea..
I prefer to get the same kits rather than buy more expensive kits for conversions... A half-dozen Monogram P-51 kits will fill the bill quite nicely for both the D and K's, and the Bs &Cs... Takes care of Dallas canopies, props, and the P-51/F-51 conversion..The A-36 is a stand-alone, though... A couple of those will allow an Apache, plus a P-51A...
You get the idea..
I also go with the same kit if there're several different variant-parts in one kit, like Monogram's P-38- (4), Mosquito -(3), Fw 1-90- (4), Hurricane- (3), HAWK\Testor's P-47D -(2), and the like...