Fix the exception "Item has already been added"

[BTMichel]

Fixes the exception "Item has already been added. Key in dictionary: 'x' Key being added: 'x'" For the Plugin Invoke-IcingaCheckDiskHealth and potentially others too. Related to #327, which has been closed, but is still an issue for us on some machines.

[cla-bot]

Thank you for your pull request. Before we can look at it, you'll need to sign a Contributor License Agreement (CLA).

Please follow instructions at https://icinga.com/company/contributor-agreement to sign the CLA.

After that, please reply here with a comment and we'll verify.

Contributors that have not signed yet: @BTMichel

Details

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Please contact us if you think this is the case.

• If you signed the CLA as a corporation, your GitHub username may not have been submitted to us. Please reach out to the responsible person in your organization.

[bobapple]

@cla-bot check

[bfenda]

Hi @bobapple
I am a colleague of BTMichel. Are there any todos open from our side for this PR.
best regards,