yesodweb/wai — comment on commit bcfa01bb3ca3710f9e0115a3bc95c1ae8886c374
File: conduit/Control/Monad/Trans/Resource.hs line 61
By gregwebs:

The ideal would be a different return value indicating it had already been freed. Farthest along these lines would be to return a Maybe with the value returned from the releasing action.