PAC : when trying to enter key members, i get this error "This resource has multiple matching values

Hello,

when trying to enter key members, i get this error "This resource has multiple matching values. Choose a unique resource from the liste of values"

i found the source of my error but don't know the plan to resolve it --> i have modified the start date of my employee before getting this error...(within project costing super user resp)

thanks in advance;