Yes and no. If they only upload wins, their MMR will be unnaturally raised. But Blizzard knows better and will continue to match them appropriately. Only having wins against players with MMR lower than the falsified one will result in smaller and smaller gains.

There is also the same 'good news' above; if other players upload some of the losses, they will be added to both profiles.

Thankfully, we've looked through a number of profiles and haven't found anyone with unusually large win streaks.