How do I prevent duplicate edges from being created between the same vertices in orientdb?

I have vertex "character" and edge "know" This is my SQL example of how to create it

CREATE CLASS Person EXTENDS V;
CREATE PROPERTY Person.name STRING;

CREATE CLASS KNows EXTENDS E;

INSERT INTO Person (name) VALUES("John")
INSERT INTO Person (name) VALUES("Ann")
INSERT INTO Person (name) VALUES("Harry")

When I create an edge between John – > Ann through

CREATE EDGE KNows FROM (SELECT FROM Person WHERE name = "John") 
TO (SELECT FROM PERSON WHERE name = "Ann")

It created it, everything is fine

However, problems arise when I accidentally create edges many times

For relationships, "know" repetition is redundant, but for others such as "access" (John [visited – >] New York), if the edge "accessed" has the attribute "date", the repetition of the edge is the desired feature

I tried to solve it by adding a unique index to the edge "knows", but then I was able to create an edge between only a pair of vertices

And it doesn't seem to me a good idea to check the edge of every existence before creation

How to solve this problem in the right way?

Solution

The direct solution is to create an index on edgeclass [out, in] To do this, you must also define the mode of the edge class:

CREATE CLASS KNows EXTENDS E
CREATE PROPERTY KNows.out LINK Person
CREATE PROPERTY KNows.`in` LINK Person
CREATE INDEX KNows.out_in ON KNows (out,in) UNIQUE