Each term in the glossary has to be unique. This is because duplicate terms in the same glossary might confuse the end user as they look for the term that has the context they need.
You can however create terms with the same name as long as they are a part of separate glossaries. Typically this is done when you have to create glossaries created on the basis of the domain/team (e.g Finance/Marketing/Product etc OR the context i.e Metrics/Projects. In this scenario - when users search for the term, even if the term has the same name, the glossary name will provide the additional context for the term.