The Broadcast parameter sends correspondence separately to each party on the work object(This is false in your case, so correct as per your requirement).
If the role given for the PartyRole parameter repeats, each such work party present will receive separate correspondence.
If this is email and there are additional parties specified (to/cc/bcc), those parties will be included on each of the separate emails sent out.If you desire one email to go out to multiple recipients, DO NOT use the PartyRole parameter.
Instead, use the To_PartyRoles, CC_PartyRoles, and BCC_PartyRoles parameters, and make sure that Broadcast is unchecked.
Also, you could review the second Java step of the activity "Corr New". You could easily identify why this repetition is happening in your case.
Hope it helps you in understanding. Kindly notify by marking this post as answered if it does for larger Pega audience and tracking purposes.