[Feature Request] Detect duplicate members of a network relation

[pyrog]

Route Bois de Senay - Place au Vin was added twice in the relation.

[vmarc]

See issue #39 (and report) for an issue that sounds more or less similar, but is in fact a lot more complex to implement.

Checking network relations for multiple members with the same route relation ids (or node ids) should be simpler.