Edge with id already exists

I am using the following to upsert an edge using Gremlin. I am following the recipe as mentioned here. This is the code I am using. The code runs in a lambda which talks to a cluster hosted in Amazon Neptune

    public void createEdge(final String id, final String label, final String fromId, final String toId, final Map<String, String> properties) {
        this.graphTraversalSource
            .V(fromId) // get vertex of id given for the source
            .as("fromVertex") // label as fromVertex to be accessed later
            .V(toId) // get  vertex of id given for destination
            .coalesce( // evaluates the provided traversals in order and returns the first traversal that emits at least one element
                inE(label) // check incoming edge of label given
                    .has(T.id, id) // also check for incoming edge of given ID
                    .where( // conditional check to check if edge exists
                        outV() // get destination vertex of the edge to check
                            .as("fromVertex")), // against staged vertex
                addE(label) // add edge if not present
                    .property(T.id, id) // with given id
                    .from("fromVertex")) // from source vertexx
            .next(); // end traversal to commit to graph

        log.info("Created edge {} if it doesn't exists between {} and {}", id, fromId, toId);
    }

The only difference from the example recipe mentioned in the tinkerpop website is this step that I have added. In my case, I want to create multiple edges between the same vertices with same label but different ID

.has(T.id, id) // also check for incoming edge of given ID

But I am getting the following exception

Caused by: java.util.concurrent.CompletionException: org.apache.tinkerpop.gremlin.driver.exception.ResponseException: 
{
    "detailedMessage": "Edge with id already exists: 123",
    "requestId": "dce46db8-1d0a-4717-a412-ee831973b177",
    "code": "ConstraintViolationException"
}

When I run the same query in gremlin console however, the experiment succeeds. The following query, how many times I execute, is returning the same edge. But the same is not happening with remote gremlin server

g.V().has('product','id','C1').as('v1').V().has('product','id','C2').coalesce(__.inE('rel').has('id', 'E1').where(outV().as('v1')), addE('rel').property('id', 'E1').from('v1')).next()

Can someone help me with this error

Thanks