@ObjectId leads to duplicate key error

Hello guys,

 if you have a pojo annotated with `@ObjectId` AND `@Id` of type `String`. You have some trouble computing the id stuff and end up with a duplicate key error, whenever you want to save twice the object as the following test case shows.

If you don't use @Id and name the field _id, the same thing happen.
If you don't use @ObjectId and replace the String by an ObjectID, it works.

Have fun.