There is no such thing as "too much sleep". We sleep because our brain cells and nerves need to regenerate, and process information, sleep is the only way we can heal and process traumatic events, body cells also need to regenerate, what is possible only during the sleep. Our immune system is perfect - but it works the best only during the sleep.
Some people need more sleep, because their health problems (often invisible and only on cellular level) and/or psychological traumas are bigger then those of the others.
Saying that somebody "sleeps too much" is human prejudice and certainly has not adverse affect on anyone`s health, on the contrary.
This prejudice make people who need more sleep feel guilty, so instead of enjoying in healing properties of sleep, they start to think that "something is wrong with them", they force themselves to stay awake, etc...and as result of that, they mess up with natural healing process and finally start do have health problems, that would never occur if they would sleep as much as they need.