From SKYbrary Wiki
A second and certified inspection by an appropriately qualified and approved Mechanic which is made during or upon completion of an aircraft maintenance task which is designated as being of sufficient safety significance to require such checking. Most such Duplicate inspections are usually made because of a requirement imposed by a Regulatory body, but may sometimes arise just from a requirement internal to a particular maintenance organisation. Careful consideration is always given to introducing a requirement for a duplicate inspection because they are viewed by many as an admission that the standards implicit in Good Maintenance Practice may not always be being followed. There is also the additional concern that a duplicate inspection should be specified at such a stage or stages in a maintenance task that it is feasible to make an objective assessment of whether a task has been correctly accomplished.