I'm a dentist.
If I understand you correctly, it's the result of one of two phenomena: 1. fusion, or 2. gemination.
Fusion is actually the fusion of two teeth in their development. They may or may not share the same root. Gemination is the partial division of a single tooth into two partial teeth (especially into two clinical crowns that appear to be fused) that share the same root.
How do you tell the difference between the two? Count the teeth. If the count is normal, then it's gemination. If the count is lower than normal, then it's fusion.
Treatment may or may not be necessary, and it should be evaluated by a doctor. He/she may decide to treat the tooth if it causes (or may potentially cause) problems.
Biologic Dental Consultant
They call it double tooth. They also used the terms fused and germinated when they diagnose a double tooth.
1: Coll Antropol. 2002 Dec;26(2):667-72.
Knezevic A, Travan S, Tarle Z, Sutalo J, Jankovic B, Ciglar I.
Department of Restorative Dentistry, School of Dental Medicine, University of Zagreb, Zagreb, Croatia.
The form of primary and permanent teeth can differ morphologically from that which is considered normal, completely or in some parts. The changes in tooth form can be hereditary or caused by some disease or trauma. Fusion is a union of one or more teeth during development. Gemination means that two separate morphological units were created by division of the tooth germ. The intention of this study was to state the prevalence of double teeth (fusion and gemination) among the persons tested, as to gender, distribution in the maxilla or mandible, and whether the anomaly occurred bilaterally or unilaterally. The results of this investigation have shown that in a total of examined 3,517 plaster models, a prevalence of double teeth was 0.2%. 57.2% of them were fusioned and 42.9% geminated.
PMID: 12528297 [PubMed - indexed for MEDLINE]
What Is A Double Tooth?
A double tooth is a tooth with a double crown. It may have one or two roots, depending on the developmental cause of the problem. A double tooth is more cavity-prone than other teeth, due to its unusual shape. In some cases, a double baby tooth is associated with a missing adult tooth.
Possible causes for double teeth include: environmental factors, crowding of tooth germs, trauma, systemic disease, and hereditary factors. Double teeth appear more frequently in children than adults. Approximately 0.5% of children have them. The front teeth are most likely to be affected by this problem. Double teeth run in families, and follow a hereditary pattern - usually autosomal dominant or autosomal recessive.
Double teeth are the result of disturbances during embryological tooth formation. Since it is not possible to know what caused the embryonic disturbance, it is not appropriate to use terms such as gemination and fusion when talking about a diagnosis.
One of the embryological causes of double teeth is gemination. Gemination is an attempted division of a tooth germ, resulting in a tooth with a bifid crown and a single root. The error occurs at the proliferative stage of tooth development.
Fusion occurs when two or more tooth germs form a single tooth. In cases of complete fusion, the tooth germs unite before the calcification process begins, and it results in a fused tooth with only one root canal. In a variant of fusion, known as concrescence, the fusion occurs at the embryonic stage when the cementum layer of the root is forming. Cases of fusion are mostly seen in the lower incisor and canine regions, and have a 78% chance of resulting in a missing adult lateral incisor.
Double teeth often have problems such as poor esthetics, dental caries, arch-length problems, and/or periodontal disease. Fused or geminated teeth usually have a vertical groove on the front and back of the teeth. These grooves may be very difficult to clean, leading to dental stains and caries. This problem can be corrected by placing a sealant or composite material into the grooves to decrease the risk of caries.
A recent dental journal article presents a clinical case of fused teeth:
Mochizuki K, Yonezu T, Yakushiji et al: The fusion of three primary incisors: Report of a case. Journal of Dentistry for Children, 66:421-425,1999.