OK, I think I have spotted why no one can reproduce your problem. You are talking about the central participant database, while everyone else is looking into the token table feature on survey basis.
Here a screenshot from the central participant datbase:
I am not sure if it makes sense to import participants to that database with the same email address. So I guess this is rather a feature than a bug.
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Stupid, I overlooked something. I can import directly into the token table of a single survey with equal e-mailadresses. I tried to import the adresses first in the participant database and then put them in the token table. It is indeed in my case easier to import hem directly in this table.