CREATE UNIQUE in neo4j produces duplicate nodes

Question

According to the neo4j documentation:

CREATE UNIQUE is in the middle of MATCH and CREATE — it will match what it can, and create what is missing. CREATE

Answer 1

Here is how you can do it with CREATE UNIQUE

MATCH (a:Person {name: 'Alice'}), (b:Person {name:'Bob'}) 
CREATE UNIQUE (a)-[:knows]->(b), (b)-[:knows]->(a)

You need 2 match clauses otherwise you are always creating the node in the CREATE UNIQUE statement, not matching existing nodes.

Answer 2

This query should do what you want (if you always want to end up with a single knows relationship between the 2 nodes):

MATCH (a:Person {name: 'Alice'})
MERGE (b:Person {name: 'Bob'})
MERGE (a)-[:knows]->(b)
RETURN a;