#collect all duplicated hard skills
duplicated_hard_skills = HardSkill.where.not(id: HardSkill.group(:name).select("min(id)"))

#go through the array of employees with duplicated hard skills and change hard_skill_id
EmployeeHardSkill.where(hard_skill_id: duplicated_hard_skills).each do  |ehs|
    original_name = HardSkill.find(ehs.hard_skill_id).name
    original_id = HardSkill.find_by(name: original_name).id
    ehs.update(hard_skill_id: original_id)
end

#delete all duplicated hard skills
HardSkill.where.not(id: HardSkill.group(:name).select("min(id)")).destroy_all