Perhaps prevent the user from accidentally adding duplicate relations.

At least during the Save Edit processing.
In fact after we add the first, it should be removed from the hints offered when making the second.
Here if I type an "A" the same item as just entered above is suggested.

@jidanni Some types of relations can have the same feature as a member multiple times, such as a road that a bus route traverses twice. Boundaries don't work like that, but I'm not sure if this is a big enough problem to spend time addressing.
Here's some more that I'm not sure how I made.

Clicking the "å·¥" also showed the same two at top.
Only upon saving did they finally become resolved into one.
@jidanni Your screenshot shows duplicate tags, not duplicate relations. Let us know if you can reproduce that problem.
(OK trying again today with the tags part, I found it very hard to get a duplicate tag tip. So the tags part is working OK.)
Closing in favor of #7648.
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@jidanni Some types of relations can have the same feature as a member multiple times, such as a road that a bus route traverses twice. Boundaries don't work like that, but I'm not sure if this is a big enough problem to spend time addressing.