Dual nationality occurs where a person is a national of two different countries. You do not normally have to give up your current nationality to become a British citizen and nor are you likely to lose your British citizenship if you acquire another nationality.
Some countries however, do not allow dual nationality and therefore, a certificate of entitlement to a right of abode will be particularly useful for applicants who belong to one of these countries. The certificate is placed in a valid passport and is only valid for the duration of that passport. Once it has expired, the applicant will need to obtain a new passport and another certificate.