please answer Sir if there are burn stain on marksheet which were put during lamination of marksheet but every information is legible. Would that marksheet be accepted during doccument verification of jobs?

Generally the laminated documents, even if it is original, it may not be recognised as original, because of the alteration done to it.

You may have to obtain a certified true copy from the department/authorities concerned to replace the original quoting the said reasons, especially if you are doubtful that the laminated cartificate may not be accepted by the employer.

Not necessary.

If your application for duplicate certificates is approved, then the one issued will be clearly marked as duplicate.