Shapeshifting is only insufficiently described as the ability to change the appearance of once body to match someone else's. It is not only the shape that changes, but the whole substance: the race, sex, and all bodily attributes are really changed.
One of the limits of shapeshifting lies in the fact, that knowledge and experience are not transfered. To be able to mirror the form of another creature the shapeshifter is required to have at least close to a similar level of experience. This difference of knowledge and experience will also stop the adventurers from mirroring the shape of the 'normal' inhabitants of Midgaardia or the other planes.
This change of form can not be maintained forever, nor can it be switched off at will. Having changed their body the shapeshifter is trapped in that ever deteriorating husk. When the substance of the mirrored form becomes too fragile to maintain, the shapeshifter is thrown back into their own true body. Death is one of the quicker ways to reduce the substance of the faked form.
A shapeshifter that is already mirroring someone else's form may choose to attempt to switch their body over to a new target. Depending on the strength of substance of their current form this will reduce the efficiency of the cloning - or the attempt may even fail.
When a shapeshifter mirrors another's form, this leaves a hidden stain on that target. This mark will fade away only slowly. While it has no effect on the target, it will reduce the efficiency of subsequent attempts to match the shape.