Exact duplicate of an appointment

[sr-gi]

TEOS accepts multiple appointments with the same locator and handles them. This is desired since not doing so would make TEOS susceptible to appointment replacement and appointment front running attacks:

Appointment replacement attack: TEOS keeps track of the latest appointment received given a locator. An attacker replaces a valid appointment by registering an invalid one after the honest party.

Appointment front running attack: TEOS keeps track of the first appointment received given a locator. An attacker prevents an appointment to be registered by an honest party by registering an invalid appointment with the same locator.

However, we may want to avoid storing identical appointments.

[sr-gi]

This is partially covered by #124. Replicas of the same appointment within the same user subscription are updates, and only the last one is kept.

Copies of the same appointment within different users are possible. This is still an issue atm, but should be solved once the subscriptions require a payment, provided that no small subscriptions are allowed.