JG Schema - addConnection seem to create duplicate connections

- Version: 0.6
- Storage Backend: Cassandra v3.11.11
- Mixed Index Backend: none
- Link to discussed bug: https://lists.lfaidata.foundation/g/janusgraph-users/message/6334
- Expected Behavior: two connections should be created for "toEdge"
- Current Behavior: four connections are created for "toEdge"
- Steps to Reproduce:
1. Creating dynamic graph and schema
```
map = new HashMap();
map.put("storage.backend","cql");
map.put("storage.hostname","cassandra");
map.put("query.force-index", "false");
map.put("schema.default", "default");
map.put("schema.constraints", "false");
map.put("graph.graphname", "transactionGraph")
ConfiguredGraphFactory.createConfiguration(new MapConfiguration(map));

graph = ConfiguredGraphFactory.open("transactionGraph");
mgmt = graph.openManagement();

transaction = mgmt.makeVertexLabel('transaction').make();
person = mgmt.makeVertexLabel('person').make();
entity = mgmt.makeVertexLabel('entity').make();

fromEdge = mgmt.makeEdgeLabel('FROM').multiplicity(ONE2ONE).make()
mgmt.addConnection(fromEdge, person, transaction)
mgmt.addConnection(fromEdge, entity, transaction)

toEdge = mgmt.makeEdgeLabel('TO').multiplicity(ONE2ONE).make()
mgmt.addConnection(toEdge, transaction, person)
mgmt.addConnection(toEdge, transaction, entity)

mgmt.commit()
```
2. Checking connections of the schema
```
mgmt = graph.openManagement()
edges = mgmt.getRelationTypes(EdgeLabel.class)
fromEdge = edges[0]
toEdge = edges[1]

fromEdge.mappedConnections().size() // as I would expect, it has two connections
toEdge.mappedConnections().size() // why 4 connections are here? I would expect only two connections similarly to the FROM edge
```
3. Note that Marc was able to reproduce the same issue also with inmemory backend (`graph = JanusGraphFactory.open('conf/janusgraph-inmemory.properties')`). See the reply from Marc here https://lists.lfaidata.foundation/g/janusgraph-users/message/6348

Reproduce the schema setup from the issue, first with the in-memory backend and then with Cassandra 3.11.11, and inspect mappedConnections() for the FROM and TO edge labels. Trace why TO reports four connections instead of two; done means both labels report the expected two connections without regressions.

Written by the indexing model from the issue text.