Ok so here is what the problem was. In the delegates tab of the user the requests were being sent on behalf of... there are a list of users who will have have rights to their calender. In properties there is a tick box that sends an email to that person for entries into the calendar. Unticking this fixed my problem! So user A was sending the request to user B, They clicked accept causing the entry to appear in their calendar. Then due to the mentioned tickbox for user A and C in user B's delegate tab. They were sent the acceptance email. Not sure if that makes sense, the user stared at me blankly when i tried to explain. Thanks for your post helped my train of thought. Hope this fixes it for you too!

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