Cognomen Jahnsen (Jahnsen 
) , globally, is quite a rare family name. The cognomen Jahnsen is characteristic of Norway, where it is quite a common surname, Denmark, where it is a rare surname, and Peru. Not in comparison with a country's population, it is commonest in Norway, the United States, and Peru. Much less frequently, Jahnsen is the forename as well as the last name.
| VARIANT | COUNTRY |
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| Jahnsen | Peru, Germany, Norway, Chile, United States |
| hans christian jahnsen Norwegian politician (b. 1823) |
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| olaf jahnsen Norwegian architect (b. 1878) |
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petter jahnsen musher (b. 1965) |
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| tore jahnsen Norwegian physician and professor (b. 1954) |
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| manuela heider de jahnsen (b. 1961) |
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| lasse jahnsen Norwegian politician |
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| frode l jahnsen researcher |
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| elisif jahnsen Norwegian association football player |
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| jan anker jahnsen researcher |
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| henrik jahnsen researcher |
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| reidun b jahnsen researcher |
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| jørgen jahnsen researcher |
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| frank jahnsen |
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| aron jahnsen |
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| synnøve økland jahnsen (b. 1981) |
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