No solution offers 100 percent protection against fraud.
Do not ever inform another person about your user ID or codes. You will never be asked by any public authority, the bank, MitId support, or others about your user ID or codes.
Suspecting abuse
If you know or suspect that your MitID is compromised, you need to block your MitID temporarily. If you have lost your MitID authenticator (phone with MitID app, code display, code reader, or chip), you can just block the missing unit.
Go to suspecting abuse
Teach yourself how to identify crank calls, false SMS, false emails and home pages (sikker digital.dk)