Prevent user from adding duplicate relations

[jidanni]

Perhaps prevent the user from accidentally adding duplicate relations.

At least during the Save Edit processing.

[jidanni]

In fact after we add the first, it should be removed from the hints offered when making the second.

[jidanni]

Here if I type an "A" the same item as just entered above is suggested.

[quincylvania]

@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.

[jidanni]

Here's some more that I'm not sure how I made.

Clicking the "工" also showed the same two at top.
Only upon saving did they finally become resolved into one.

[quincylvania]

@jidanni Your screenshot shows duplicate tags, not duplicate relations. Let us know if you can reproduce that problem.

[jidanni]

(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.)

[quincylvania]

Closing in favor of #7648.