We have been observing this for quiet some years now, where a candidate negotiates one offer letter CTC with the other and takes advantage of the same.
Some companies for the same reason always do not share a hard copy of the offer letter and do regular follow ups for the same.
Some even keep hunting for resources even till last minute of the candidate joining as plan B.
This is an overall challenge for all of us, in current scenario where demand is very high for some positions that head hunting is also very difficult.
Some group of companies has the following options to track the same:
A unified portal to track the applicants history- Example NASSCOM has a portal for the same reason where feedback over the candidate is listed.
Some back ground verification companies update this list and even shares with the employer - the number of times a person has denied offers etc.,
There several groups which are run by group of HRs and Recruitment agencies where the same feedback is updated.
But to be frank there is no strict mechanism for the same and it’s always advised to have a back up / Plan B candidate ready in place.