Swedish names in plural?
Is there a way to use names in the plural in Swedish, e.g. if you're talking about multiple people with the same name? Like: "I know two Martins."
Is there a rule for this, or do they simply not use plural in such cases?
The name doesn't change. Sometimes some people are a bit playful though and say for example "jag känner två Johannor" instead of "jag känner två Johanna". At least where I come from!
Normally the name does not change, but some names do, as 'en Maria, två Marior', as the Johanna, mention by lennonmacca, you could probably take that as a rule for all names ending in '-a' becoming '-or'. It's because of the feminine ending, as in the word 'katta' (female cat), in plural 'kattor', 'svenska' (a female swede) becoming 'svenskor'.
But, 'En Martin' just becomes 'Två Martin', or it should be 'två Martinar', as you could say, everyone would understand and it is correct, used sometimes, it is also okey to pluralise if you say 'de två Martinorna'. Historically and locally you could also say 'de tvenne Martinarna', 'Martina tu' and more variants, which could be used, primarly in literature, today.
Also 'en Sven' becomes 'två Svennar', but that has also a secundary meaning, meaning 'swedes' if spelled 'svennar', sometimes in a negative context (even thou the word only mean 'boy, youth', 'servent', or historically 'low nobelman').
As lennonmacca has said. The "correct" way is to say Vi har två Maria i klassen. But it is fun to break rules and in spoken language it is common to say Vi har två Marior i klassen.
Svennar is a derogatory word for "ethnic swedes" used in slang among immigrants. If someone said Vi har två svennar i klassen it would always be understood to mean we have two ethnic swedes in the class.