Debian: GPG Error During apt-get update

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W: GPG error: http://security.debian.org testing/updates Release:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY key-number

Is this type of message appearing? It's quite annoying, especially since it happens every year. To solve this problem, follow these steps:

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gpg --keyserver pgpkeys.mit.edu --recv-key key-number
gpg -a --export key-number | apt-key add -