When creating a new contact for the same or similar name, or updating an existing contact, if all the fields in the contact do not match, or are not a subset of the other, a duplicate contact is created - see below

Case 1:John D Doe, 770-555-1335John D Doe, 770-555-1335Exact match. Merged.Case 2:John D Doe, 770-555-1212John D Doe, 770-555-1335Phone numbers mismatch. Neither is a subset of the other. Duplicated.Case3:John D Doe, 770-555-1212John Doe, 770-555-1212Second is a subset of the first. Merged.Case 4:John D Doe, 770-555-1212John D Doe, 770-555-1212, 770-555-1335First is a subset of the second. Merged.

Case 5:John D Doe, 770-555-1212John Doe, 770-555-1212, 770-555-1335Neither matches a subset of the other. Duplicated.Case 6:John Doe, 770-555-1212John D Doe, 770-555-1212, 770-555-1335Everything in the first appears in the second. Merged. 

This topic is valid for:

Nokia Lumia Icon,Nokia Lumia 1520,Nokia Lumia 1320,Nokia Lumia 1020,Nokia Lumia 928,Nokia Lumia 925,Nokia Lumia 920,Nokia Lumia 900,Nokia Lumia 830,Nokia Lumia 822,Nokia Lumia 820,Nokia Lumia 810,Nokia Lumia 800,Nokia Lumia 735,Nokia Lumia 635,Nokia Lumia 630,Nokia Lumia 620,Nokia Lumia 530,Nokia Lumia 521,Nokia Lumia 520,Nokia C6,Nokia C6,Nokia C5,Nokia C3,Nokia C2,Nokia E73,Nokia E72,Nokia E71,Nokia E71x,Nokia E7,Nokia E63,Nokia E6,Nokia E5,Nokia N97,Nokia N97,Nokia N95,Nokia N95 8GB,Nokia N900,Nokia N8,Nokia X6-00,Nokia X3,Nokia X2-01,Nokia 808 PureView,Nokia 700,Nokia 6790,Nokia 6350,Nokia 5800,Nokia 5310,Nokia 5230,Nokia 5130,Nokia 2720,Nokia 2330,Nokia 2320,Nokia 1616