Assume that you have contacts in Lync 2013 who have at least one telephone number in their contact card. You sign in to Lync 2013, and then you open a contact card of one of the contacts. In this situation, a telephone number is displayed two times in the contact card.
Note The telephone number that is displayed in the contact card may be an E164 number or an extension.
This issue occurs because Lync 2013 aggregates the contact card data from multiple sources, and sometimes different formats of the same telephone number are interpreted as separate entities.
To resolve this issue, install the following update:
2825630 Description of the Lync 2013 update 15.0.4551.1005: November 7, 2013