Building on your past work or even repurposing it as a cost-cutting measure isn't unheard of. I'd argue that it's not inherently that big of a deal, really -- unless people catch on. Then it's just becomes funny, or sad, or perhaps a bit of both.
With some Call of Duty fans down on Infinity Ward for not doing all that it could to reinvigorate the series, this side-by-side video comparison of a scene from Ghosts and one from the ending of Modern Warfare 2 couldn't arrive at a better time. Cannot unsee.