I just dont think a player under contract should be available due to an error. Mistakes will happen, it's part of the manual process. No way to get around that.
If a player is under contract as shown by the forum history and the s/s, it should not matter where else it shows him as available. If a player ends up on two teams (similar to what happened in SACd last year with Deablo), the forum history will tell a story regarding where he needs to be and a correction happens. I agree it kind of sucks, but things happen and we follow the trail to resolve the issue.
I also use the MFL site to find FAs, but I usually do a quick search in the s/s to verify FA status.
Now I do not verify players already out in the bidding process, so if someone else posts a player, then I could be in trouble and get caught winning a player under contract.
Just my 5 cents (used to be 2 cents, but inflation caused an increase)