Double voting

[dnkolegov]

The following attack may be possible:

1. A malicious validator A submits a checkpoint Ch_A, the subnet actor counts its votes.

2. Validator A leaves the subnet, receives all the staked tokens back, and transfers them to another malicious validator B.

3. B adds the stake, and then B may vote for the same checkpoint Ch_A. As a result, A's weight is used twice.