Hello Angiolo,
Lost likely it means exactly what it says: "a certificate with the same label or public key already exists in the certificate store.". i.e. most likely you have already imported that certificate into the cert store in the past. It happened to me too when I accidentally clicked on the import button for the second time.
Go the the store so you can see its content WITHOUT any filters applied. Then check the serial numbers/keys as well as labels of the certificates already present.
Hope this helps,
Roman
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Roman Chloupek
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