A "twin" refers to one of two offspring produced by the same pregnancy. Twins can be classified as either identical or fraternal. Identical twins, or monozygotic twins, develop from a single fertilized egg that splits into two embryos, sharing the same genetic material. Fraternal twins, or dizygotic twins, result from two separate eggs being fertilized by two different sperm cells, leading to genetically unique siblings. In various contexts, the term "duplicate" can refer to an exact copy of an object or document. For example, a duplicate key is a copy of an original key, allowing access to the same lock. Duplicates are often used in data management to ensure backup and redundancy, helping to prevent data loss.