Carrying two copies of an allele is disadvantageous.
Sickle cell disease (SCD) is caused by having two genes for "Hemoglobin S," which distorts the shape of red blood cells into a crescent, or sickle, shape.
Heterozygotes only carry one copy of it; a typical "Hemoglobin A" gene and a "Hemoglobin S" gene: they have 'sickle cell trait' (SCT), but not the disease. Having SCT means that a person has inherited the sickle cell gene from one of his or her parents. People with SCT usually do not have any of the symptoms of SCD and live a normal life.
Sources suggest that carriers of the "Hemoglobin S" gene are resistant to Malaria, which is quite deadly in warm, tropical locations like Africa and the Mediterranean.
In areas with high rates of Malaria infection, people without sickle cell trait are likely to fall ill, and possibly succumb, to Malaria. Those with sickle cell trait are immune, and more robust than either those without the trait, or those who have sickle cell disease.
Here's what you should know about sickle cell trait from CDC.gov.