When a user submits a request and it is pending approval, the account will still be shown as available for other users. If another user then submits a request, it will be rejected once the first request has been approved.
An enhancement request has been submitted to make accounts show as unavailable while an account is pending approval. This enhancement has been submitted as BFER 4610.
This request will be reviewed by Product Management for possible inclusion in a future version. There is currently no ETA for when this may be implimented.