Reject payment accounts with duplicate names [0.25 XMR]

[woodser]

This issue requests rejecting payment accounts with previously given names.

A test should be added to test correct error handling in that case.

[github-actions[bot]]

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[klinki]

Please assign it to me, I will look into it.

[woodser]

I've assigned the issue to you. Thanks.

[woodser]

Fixed with #556 and #180.