The number sequence is given to the records created and same number sequence i have shown in the look up field in the next form relating to all records.
The thing which i want is as number sequence is already alloted to the records in the first form and is shown in next form,but if a record for the number sequence is generated in the next form,then for the creation of next record that alloted number sequence should not be displayed again in that look up field.
Actually to be short and sweet,if any number sequence hase been applied to any of the records from look up,next time when user creates record,that already allocated number sequence should not be displayed in the look up.