can i assign a labor resource more than once to the same activity?
or, for every person we have, we need to have a different labor?


i mean i wanted to tell ms project that an activity will have 5 labor. but couldnt do it.

No. Each can only be assigned to a task once. If you need more than one labor resource, either increase the resource's max. units to 200% and assign 200%, or create another resource and assign the additional resource.

ok. but if i do 200% it creates a problem when leveling later as ms project will try to level it down to 100% correct?

No. If you increase the max. units to 200%, the resource will not be over-allocated. Leveling is only for circumstances where Peak Units > Max. Units.