Label duplication

[vsemozhetbyt]

If a collaborator adds labels before PR creation, the bot may add the same labels after creation. This is a harmless no-op, but duplicate log messages increase noise in the conversation.

Example: nodejs/node#12381 (see two initial label comments).

Maybe a check for duplication can be added.

[phillipj]

Closing this for now as it's a duplicate of #75. I have just contacted GitHub support about this, as it would have been best if we can avoid double checking what labels already exists before adding labels.