Merging ways may result in duplicate relation membership

[DujaOSM]

When ways being merged participate in the same relation but with different roles, iD will happily duplicate the relation membership in the combined way, keeping the roles intact:

1. Create a way A, and add it to a (for example) multipolygon M with no role
2. From A's endpoint, create a way B, and add it to M with role "outer"
3. Shift-Select A and B and [C]ombine them.

As a result, the combined way participates in M twice, once as "outer" and once with no role. Similar will happen of one is outer and the other inner, or if the relation is a route.

While there's no hard prohibition against duplicate relation membership, this is very rarely needed, and usually inadvertent (for multipolygons in particular, I don't think it's ever valid). The above operation results in an invalid multipolygon which does not render in Carto, and iD currently does not provide validation against such situation.

Suggested solution:

1. When merging such ways and there is no role conflict, add it to a relation once, with the provided role
2. If there is a role conflict, prevent merging (similar as with tag conflicts)

As a bonus, add a warning against any duplicate relation membership (sometimes valid in route relations, but then mapper can ignore the warning).

[DujaOSM]

May I gently ping this? It seems to have slipped through the cracks. Now that it's possible to multi-select ways to add to relations, the problem is even more pronounced -- if I had already added a way to a relation, it's now quite easy to inadvertently re-add it.

[sevku]

noticed the same issue 👍