It depends on the character. I'm happy with my GOTG Star Lord and AoU Scarlet Witch. I won't be buying any "dupes" of those, even if the costumes change. I currently own three Caps (TFA Rescue, TFA Hero Costume, and TWS Stealth) and plan on picking up the Civil War figure as well. When it comes to the Caps, I wanted at least one from each of his solo films and for the outfits/styling to be significantly different from one another. For those particular figures, they are. The Avengers Cap and AoU Cap are far too flashy for my taste
I also have an Avengers Loki and a TDW Loki, although I half wish I had stuck with just the Avengers figure because the outfits are so similar. At this point, it's unlikely that I'll pick up Ragnarok Loki if he looks relatively unchanged (I assume there will be small cosmetic changes, but not enough for me to justify shelling out another $250).
On the other hand, I'll buy any and all Bucky/Winter Soldier figures they put out, even with similar outfits. There are fewer figures for him (still hoping for a
very late release of a TFA Bucky) so I know he'll be able to fit into my case and my wallet.
For characters I'm not crazy about, I'll pass completely, especially when their costumes look virtually the same with each release (ex Black Widow).