Since 1 July 2001 Sweden has allowed dual (and multiple) citizenship. This means two things: you normally do not have to renounce your original citizenship to become Swedish, and a Swedish citizen who acquires another country's citizenship does not automatically lose their Swedish one.
Sweden cannot, however, change other countries' rules. If your country of origin does not allow dual citizenship, that country may withdraw yours automatically. So check both the Swedish position with Migrationsverket and your country of origin's rules with their authorities before you apply.
Sources
- The Government of Sweden — migration and asylum policy
- Migrationsverket — becoming a Swedish citizen
- Riksdagen — the Swedish Citizenship Act
Official details can change. Always confirm what applies to your own case directly with the responsible authority.