We are filing a new patent, on one concept but there is a provisional patent available for public which is not valid (may be they have not paid the fee or it was withdrawn). Once our patent is approved, can someone still come behind us based on that provisional patent?
Normally provisionals are only published if they are used as a priority document for a non-provisional, so I think you might have missed something. If you have not, the time to claim priority on a provisional application is one year after filling, after that, it cannot be claimed anymore.
But there is another issue here, published provisionals count as prior art, if you are that concerned about somebody else following up on this patent, have you checked that it won't stop your application by rendering it not novel or not inventive?