SOLVED: C#.NET X509Certificate: System.InvalidOperationException: 'An X509Extension with OID '2.5.29.15' has already been specified.'

You may see the error "System.InvalidOperationException: 'An X509Extension with OID '2.5.29.15' has already been specified.'" when creating a self signed certificate.

This can occur if you try and add multiple KeyUsageFlags as two separate extensions.

certificateRequest.CertificateExtensions.Add(new X509KeyUsageExtension(X509KeyUsageFlags.KeyEncipherment , true));

certificateRequest.CertificateExtensions.Add(new X509KeyUsageExtension( X509KeyUsageFlags.DigitalSignature, true));

To resolved the issue assign the flags to a single extension.

certificateRequest.CertificateExtensions.Add(new X509KeyUsageExtension(X509KeyUsageFlags.KeyEncipherment | X509KeyUsageFlags.DigitalSignature, true));





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