Question about RBC

I'm looking at the on_propose function in sailfish-rbc/src/abraham/worker.rs.

My understanding is that this is called to process any proposal.
We first look up messages based on the round number of the proposal digest.
We then look up the tracker in messages.map based on the digest itself.
If this is the first time we see this digest, then we will create a new tracker and then broadcast a vote for it.

It seems to me that a malicious party could make many proposals in a single round, and an honest party would carry out the above for each of them.
This isn't right: for each round, a party should only vote for a single proposal per party per round.

I may very well be misunderstanding something, so please feel free to clarify things.
Thanks!