fix JENKINS-46940 by preventing entries having the same project name from being added more than once

[dliakh]

JENKINS-46940

In addition to the duplicate protection added earlier (that didn't work for some reason)
also prevent duplicates by checking whether there is an entry having the same project name already.
(Tested on the local Jenkins setup which had an issue with duplicated before the change and doesn't have it after)
Please, can you check whether this technique (preventing duplicates based on the name of the project associated with the entry)
is ok

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[dliakh]

Well, need to recheck how that affect [possible] cases of having multiple projects with the same name
(is that possible when using Jenkins' "folders", for example?)

[akrapfl]

Wonder what the checks failure means, and if it happened before this PR.

@dliakh I haven't looked at your changes yet, but I'd like to test this. In response to your concerns over side effects, I wonder if it would be possible to add a configuration flag somewhere to make the deduplicating behavior optional. Defaults to off, and turn it on if you want that correction?