cannot add ssh key: fingerprint already taken

Summary

(another variation on the theme, this is on gitlab.com)

Adding a second id_rsa.pub to a hosted gitlab.com account fails with "fingerprint already taken"

Steps to reproduce

1. create a user account
2. add an ssh rsa key from plesk/wordpress "production" instance
3. add an ssh rsa key from plesk/wordpress "development" instance -- fail.

The keys are different. The account has noting else, just the first key.

What is the current bug behavior?

The big red box pops up:

The form contains the following error:

    Fingerprint has already been taken

What is the expected correct behavior?

2nd key should be added