This may sound silly, but is this normal for her age? My DD is usually very cautious with her toys and when she sets stuff down she is very gentle. But for some reason with just her doll she picks her up and throws her as hard as she can on the doll bed. We've been practicing "putting dolly to bed" and I show her over and over how to do it GENTLY and she still picks the doll back up and throws her very hard back down. DD is a very smart kid and I'm sure she understands what I am telling her as I normally only have to tell her something once.
The reason I am concerned is that I'm expecting another baby in March and I'm worried that she'll be violent with the new baby, that is why I've been introducing the baby doll and how to be gentle with her so she kinda understands what's going on when the real baby comes. Is this just a faze she's going through? Any advice/tips/input wouls help! Thanks:)

It sounds like an 'independence' sort of issue. Do you normally tell her to 'be gentle' with her toys, or only this one doll? If that is the case, you have probably aroused her contrary curiosity "Why does THIS toy have to be put to bed gently? What will happen if I don't" or "Thanks for the advice, now I will just do it MY way!" Who knows what goes through their little heads. I don't think it is too serious though, she probably doesn't really relate to the doll being a "baby" yet.

she might just like the extra attention she gets when she's "violent" to her baby (good, bad, serious, teaching... whatever... it's all attention!)! i can't even begin to tell you all the weird things my three year old does over and over again just because it gets some sort of a reaction out of us!

unless she's in daycare, or has a regular sitter/nanny (where it could be a learned behavior), i wouldn't worry about it.

Not to worry, mine does the same thing! At first I was worried cause I too am having another baby in December, but I spoke to a friend of mine who is a child psychologist and she reassured me its only a phase. I guess children are sometimes jealous or just want the attention. My DD takes her doll and hits her, throws her, the other day she took the doll threw it out the door and told her she was naughty. Now she hates it and tells me she is afraid of it.
When awe are around real babies she is so careful with them and so nurturing its amazing. I love the way she gets worried if she hears a baby crying in the store and thinks the baby is hungry or needs a hug. Its very sweet.
But don't worry but keep telling her that you have a baby in your belly that will be coming to stay with you soon. I also read somewhere that bringing a gift home with the baby for her will help. Like "oh, look what the baby brought just for you"

I think its normal for some reason my 16mo DD does the same thing with her dolls. And she also gets a mean streak with us when she doesnt get her way and Im a stay at home mom with no babysitters and shes never out of my sight so I dont know where she gets this from but I guess its just there personalities coming thru.