This occurs if a participant joins the meeting with the same "from:" SIP address as another participant already in the Lync/Skype for Business meeting. For instance, this will occur with PSTN participants who have a shared or anonymized CallerID and the PSTN gateway creates their SIP From: address from the incoming call. The call drop is initiated by the AVMCU.
To avoid this, route telephone participants joining Skype/Lync meetings through the PSTN conferencing/Microsoft Mediation Server rather than through the Meeting Server Dual Homing gateway.
The AVMCU enforces a limitation that directly connected participants must have unique From: address values. When a duplicate appears, the AVMCU believes the new call to be the same participant as the existing call and drops the prior call. The Meeting Server does not rename or transform participant's identification, so potential conflicts are not filtered/stopped before hitting the AVMCU.
The Microsoft Mediation Server role used in Skype/Lync PSTN conferencing mitigates the conflict for participants connecting through it to the AVMCU.