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.

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[BTMichel]

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

CLA has been signed

[BTMichel]

Will create a new PR since the CLA check does not seem to be re-triggered

[BTMichel]

Created #424 but CLA check does not recognize my username apparently, will check how to proceed